Consolidate Autotools linux files

There are many architecture-specific linux files in the config
directory, all of which simply redirect to linux-gnulibc1.

This change renames linux-gnulibc1 to linux-gnu and deletes the more
specific files.

config/README.md

@@ -32,8 +32,6 @@ An Autotools build will first use `$host_cpu`, `$host_os`, etc. to try to find a
 suitable platform file in `config` to source and start checking compilers. The
 code that does this is in `configure.ac` (search for `host_os`). For example,
 MacOS will source the `apple` file and FreeBSD will source the `freebsd` file.
-There are a bunch of Linux files, but they all eventually invoke
-`linux-gnulibc1`.
 
 If you dig into one of these files, the way that they check for compilers is
 rather crude. Each OS script will simply source the various C, C++, and

config/cygwin

@@ -17,5 +17,5 @@
 #
 # See BlankForm in this directory for details.
 
-# Use the linux-gnulibc1 file.
-. $srcdir/config/linux-gnulibc1
+# Use the linux-gnu file
+. $srcdir/config/linux-gnu