gprofng: Don't hardcode -Wno-nonnull-compare

Use -Wno-nonnull-compare only if it is supported.

	PR gprof/28969
	* libcollector/Makefile.am (AM_CFLAGS): Replace
	-Wno-nonnull-compare with GPROFNG_NO_NONNULL_COMPARE_CFLAGS.
	* libcollector/configure.ac (GPROFNG_NO_NONNULL_COMPARE_CFLAGS):
	New AC_SUBST for -Wno-nonnull-compare.
	* libcollector/Makefile.in: Regenerate.
	* libcollector/aclocal.m4: Likewise.
	* libcollector/configure: Likewise.