Repurposed macro: AX_CFLAGS_WARN_ALL_ANSI becomes (minus warnings) AX_CFLAGS_FORCE_C89

[Reuben Thomas]

I attach a simplified version of this obsolete macro that rescues the
useful bit of functionality that is not available elsewhere: it forces
C89 mode. This is logically separate from enabling warnings, and it's
better than AC_PROG_CC_C89, which only ensures the compiler will
compile at least C89, rather than requiring strict conformance.

I want to use this myself for GNU Zile. I tried using
AX_CFLAGS_WARN_ALL_ANSI with it before and ran into problems with
gnulib, but that's a separate issue; I want if at all possible to get
it compiling in C89 mode because if I don't then I make errors that
only show up on compilers that are strict by default, and those come
to me as bug reports, not test failures :(

If you like it, I'll add it.

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ax_cflags_force_c89.m4
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