Add -frandom-seed=RepeatabilityConsideredGood to CFLAGS for building this

library. As the value suggests, this allows the library to be built repeatably;
without this flag, gcc uses a random value in its parsing.

Since the random seed is only used when handling files which do not have any
externally-visible symbols, this change is not needed for any other libraries
in the FreeBSD base system.

Discussed on:	freebsd-arch (in early November)
Approved by:	kan
MFC after:	1 week

gnu/lib/libstdc++/Makefile

@@ -13,6 +13,7 @@ SHLIB_MAJOR=	4
 
 CFLAGS+=	-DIN_GLIBCPP_V3 -DHAVE_CONFIG_H
 CFLAGS+=	-I${.CURDIR} -I${SUPDIR} -I${GCCDIR} -I${SRCDIR}/include
+CFLAGS+=	-frandom-seed=RepeatabilityConsideredGood
 CXXFLAGS+=	-fno-implicit-templates -ffunction-sections -fdata-sections \
 		-Wno-deprecated