* lib/strerror_r.c: Use STRERROR_R_CHAR_P to decide whether the
system strerror_r returns char *, and HAVE_DECL_STRERROR_R to
decide whether it either does that or returns an integer. In the
former case, use the system strerror_r even on platforms like
Android API level 23 that don’t have __xpg_strerror_r; also
check for strerror_r failure just in case.
* m4/error.m4 (gl_PREREQ_ERROR):
* m4/strerror_r.m4 (gl_PREREQ_STRERROR_R):
Use system extensions on Android, to avoid mishandling strerror_r
on API level 23 and later.
* modules/error, modules/strerror_r-posix (configure.ac):
Surround gl_PREREQ_ERROR with AS_IF instead of plain if, so that
AC_REQUIREs are propagated out.