meson: Fix the gnulib checks for isnan* functions
The gnulib math code uses __builtin_isnanf
and __builtin_isnanl
within the __GNUC__ >= 4
ifdef, and clang doesn't provide those
builtins, only the one without a suffix. Make the meson check
match the code it controls, using the exactly right builtins.
Set REPLACE_ISNAN
to 1 if either of the have_isnan* functions failed,
this matches how gnulib's m4 routines does it (in gnulib/m4/isnan.m4).
This fixes the isnan
functions in the gnulib math header replacement
work on Clang.