strerror_r-posix: Fix override of AC_FUNC_STRERROR_R
Requiring autoconf's AC_FUNC_STRERROR_R and then calling AC_DEFINE
ourselves leads to two insertions of #define STRERROR_R_CHAR_P
in confdefs.h, but with different values (at least on glibc). This
in turn sparks a gcc warning about redeclaring the macro, and kills
any subsequent tests that use -Werror (which now error because of
the conflicting define, whether or not the test used to error).
A better approach is to replace the autoconf macro with a version
of our own that doesn't probe anything, so that the AC_DEFINE is
reached exactly once.
* m4/strerror_r.m4 (AC_FUNC_STRERROR_R): New override.
(gl_FUNC_STRERROR_R): Don't reuse AC_DEFINE() with a potentially
different value.