Replaced an ugly hack to selectively disable warnings

in contributed sources with just a hack made possible
by bsd.sys.mk,v 1.33.  This is better because it just
nulls out the warning flags rather than adding gcc(1)
specific -w option to CFLAGS.

lib/libc/Makefile

@@ -97,3 +97,6 @@ libkern.${MACHINE_ARCH}:: ${KMSRCS}
 .endif
 
 .include <bsd.lib.mk>
+
+# Disable warnings in contributed sources.
+CWARNFLAGS:=	${.IMPSRC:Ngdtoa_*.c:C/^.+$/${CWARNFLAGS}/}

lib/libc/gdtoa/Makefile.inc

@@ -19,6 +19,3 @@ CLEANFILES+=gdtoa_${src}
 gdtoa_${src}:
 	ln -sf ${.CURDIR}/../../contrib/gdtoa/${src} ${.TARGET}
 .endfor
-
-# XXX An ugly hack to disable warnings for gdtoa sources.
-CWARNFLAGS+=   ${.IMPSRC:Mgdtoa_*.c:C/^.+$/-w/}