meson: pass -fno-builtin when testing whether stpcpy is a builtin
meson: pass -fno-builtin when testing whether stpcpy is a builtin

In https://bugzilla.gnome.org/show_bug.cgi?id=794555 the tests for
posix_memalign and stpcpy were extended to catch the case where
the compiler provides an incomplete builtin.

Under MSYS2 the example code still compiles and links while the real usage
of stpcpy fails to build. To prevent the MSYS2 gcc from using the builtin
versions pass -fno-builtin.

https://bugzilla.gnome.org/show_bug.cgi?id=793729

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib