Fix `configure' helpstring for older version of bash

[albinahlback]

On Debian Buster the helpstring did not display all options. This commit fixes it.

Could someone with a newer version check that this prints the helpstring OK? So just do

$ ./bootstrap.sh
$ ./configure --help

and check that it looks ok.

[edgarcosta]

I am looking into this

[edgarcosta]

# neofetch
       _,met$$$$$gg.          root@3031627062cc 
    ,g$$$$$$$$$$$$$$$P.       ----------------- 
  ,g$$P"     """Y$$.".        OS: Debian GNU/Linux 10 (buster) x86_64 
 ,$$P'              `$$$.     Host: BHYVE 1.0 
',$$P       ,ggs.     `$$b:   Kernel: 5.15.49-linuxkit 
`d$$'     ,$P"'   .    $$$    Uptime: 16 mins 
 $$P      d$'     ,    $$P    Packages: 195 (dpkg) 
 $$:      $$.   -    ,d$$'    Shell: bash 5.0.3 
 $$;      Y$b._   _,d$P'      CPU: Intel i7-9750H (2) @ 2.600GHz 
 Y$$.    `.`"Y$$$$P"'         Memory: 621MiB / 7859MiB 
 `$$b      "-.__
  `Y$$                                                
   `Y$$.
     `$$b.
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              `"""

# ./bootstrap.sh
autoreconf: Entering directory `.'
autoreconf: configure.ac: not using Gettext
autoreconf: running: aclocal --force --warnings=all 
autoreconf: configure.ac: tracing
autoreconf: configure.ac: creating directory config
autoreconf: configure.ac: not using Libtool
autoreconf: running: /usr/bin/autoconf --force --warnings=all
autoreconf: running: /usr/bin/autoheader --force --warnings=all
autoreconf: configure.ac: not using Automake
autoreconf: Leaving directory `.'
# ./configure --help
`configure' configures FLINT 3.0.0-dev to adapt to many kinds of systems.

Usage: ./configure [OPTION]... [VAR=VALUE]...

To assign environment variables (e.g., CC, CFLAGS...), specify them as
VAR=VALUE.  See below for descriptions of some of the useful variables.

Defaults for the options are specified in brackets.

Configuration:
  -h, --help              display this help and exit
      --help=short        display options specific to this package
      --help=recursive    display the short help of all the included packages
  -V, --version           display version information and exit
  -q, --quiet, --silent   do not print `checking ...' messages
      --cache-file=FILE   cache test results in FILE [disabled]
  -C, --config-cache      alias for `--cache-file=config.cache'
  -n, --no-create         do not create output files
      --srcdir=DIR        find the sources in DIR [configure dir or `..']

Installation directories:
  --prefix=PREFIX         install architecture-independent files in PREFIX
                          [/usr/local]
  --exec-prefix=EPREFIX   install architecture-dependent files in EPREFIX
                          [PREFIX]

By default, `make install' will install all the files in
`/usr/local/include' and `/usr/local/lib' etc.
You can specify an installation prefix other than `/usr/local'
using `--prefix', for instance `--prefix=$HOME'.

For better control, use the options below.

Fine tuning of the installation directories:
  --bindir=DIR            user executables [EPREFIX/bin]
  --libdir=DIR            object code libraries [EPREFIX/lib]
  --includedir=DIR        C header files [PREFIX/include]

Optional Features:
  --disable-option-checking  ignore unrecognized --enable/--with options
  --disable-FEATURE       do not include FEATURE (same as --enable-FEATURE=no)
  --enable-FEATURE[=ARG]  include FEATURE [ARG=yes]
  --enable-pthread        Enable pthread [default=yes]
  --enable-reentrant      Build reentrant version of library [default=no]

  --enable-thread-safe    Enable thread-local storage [default=yes]
  --enable-assert         Enable use of asserts [default=no]
  --enable-debug          Compile FLINT with debug information [default=yes]
  --enable-coverage       Enable test coverage [default=no]
  --enable-cxx            Enable C++ wrappers, including tests [default=no]
  --enable-dependency-tracking
                          Enable GCC automated dependency tracking
                          [default=yes]
  --enable-gmp-internals  Enable calling GMP internals directly [default=yes]
  --enable-avx2           Enable AVX2 instructions [default=no]
  --enable-avx512         Enable AVX512 instructions [default=no]

Optional Packages:
  --with-PACKAGE[=ARG]    use PACKAGE [ARG=yes]
  --without-PACKAGE       do not use PACKAGE (same as --with-PACKAGE=no)
  --with-gmp-include=DIR  GMP include directory
  --with-gmp-lib=DIR      GMP library directory
  --with-gmp[=DIR]        GMP install directory
  --with-mpfr-include=DIR MPFR include directory
  --with-mpfr-lib=DIR     MPFR library directory
  --with-mpfr[=DIR]       MPFR install directory
  --with-blas-include=DIR Use BLAS and specify its include directory
  --with-blas-lib=DIR     Use BLAS and specify its library directory
  --with-blas[=DIR]       Use BLAS and specify its install directory
  --with-gc-include=DIR   Use GC and specify its include directory
  --with-gc-lib=DIR       Use GC and specify its library directory
  --with-gc[=DIR]         Use GC and optionally specify its install directory
  --with-ntl-include=DIR  Use NTL and specify its include directory
  --with-ntl-lib=DIR      Use NTL and specify its library directory
  --with-ntl[=DIR]        Use NTL and optionally specify its install directory

Some influential environment variables:
  CXX         C++ compiler command
  CXXFLAGS    C++ compiler flags
  LDFLAGS     linker flags, e.g. -L<lib dir> if you have libraries in a
              nonstandard directory <lib dir>
  LIBS        libraries to pass to the linker, e.g. -l<library>
  CPPFLAGS    (Objective) C/C++ preprocessor flags, e.g. -I<include dir> if
              you have headers in a nonstandard directory <include dir>
  TESTCFLAGS  Choose different C compiler flags for tests [default=CFLAGS].
              Can be useful to compile library at a specific optimization
              level while compiling tests fast.
  ABI         Desired ABI
  LDCONFIG    ldconfig tool
  CC          C compiler command
  CFLAGS      C compiler flags
  CPP         C preprocessor
  CXXCPP      C++ preprocessor

Use these variables to override the choices made by `configure' or to help
it to find libraries and programs with nonstandard names/locations.

Report bugs to <https://github.com/flintlib/flint2/issues/>.
FLINT home page: <https://flintlib.org/>.
#