Remove custom code for checking compiler warnings, using
gl_WARN_ADD instead. Don't list all flags ourselves, use
gnulib's gl_MANYWARN_ALL_GCC to get all possible GCC flags,
then turn off the ones we don't want yet.

* acinclude.m4: Rewrite to use gl_WARN_ADD and gl_MANYWARN_ALL_GCC
* bootstrap.conf: Add warnings & manywarnings
* configure.ac: Switch to gl_WARN_ADD
* m4/compiler-flags.m4: Obsoleted by gl_WARN_ADD