Use -Wold-style-definition.

This patch makes glibc build with -Wold-style-definition to avoid
old-style function definitions creeping back in by accident.

Tested for x86_64 and x86 (testsuite, and that installed shared
libraries are unchanged by the patch).  Also tested build for arm,
mips64 and powerpc32.  Hopefully there aren't any remaining
system-specific files with old-style definitions whose formatting
evaded my searches, but if there are, they will be easy to fix.

	* Makeconfig (+gccwarn-c): Add -Wold-style-definition.
	* Makefile ($(objpfx)c++-types-check.out): Filter out
	$(+gccwarn-c) instead of -Wstrict-prototypes.