autogen.sh: successfully detect if the autoreconf program is installed

The "which" utility is not guaranteed to be installed either, and if it
is, its behavior is not portable either. This means that when autoreconf
is installed, the `which` check will report a fatal error because the
which tool did not exist and the shell returned a nonzero status when
attempting to fork+exec. If it did exist, it might not be an
implementation of `which` that returns nonzero when commands do not
exist.

The general scripting suggestion is to use the "command -v" shell
builtin; this is required to exist in all POSIX 2008 compliant shells,
and is thus guaranteed to work everywhere.

For some in-depth discussions on the topic, see:
- https://mywiki.wooledge.org/BashFAQ/081
- https://unix.stackexchange.com/questions/85249/why-not-use-which-what-to-use-then/85250#85250

Examples of open-source shells likely to be installed as /bin/sh on
Linux, which implement the 15-year-old standard: ash, bash, busybox,
dash, ksh, mksh and zsh.

Signed-off-by: Eli Schwartz <[email protected]>

autogen.sh

@@ -36,7 +36,7 @@ cd "$srcdir"
 
 mkdir -p m4
 
-AUTORECONF=`which autoreconf`
+AUTORECONF=`command -v autoreconf`
 if test -z $AUTORECONF; then
     echo "*** No autoreconf found ***"
     exit 1