configure.ac: Change default of poststage1_ldflags to be empty if poststage1_libs is set.

./:	* configure.ac: Change default of poststage1_ldflags to be empty if
	poststage1_libs is set.  When poststage1_libs is empty, and
	ENABLE_BUILD_WITH_CXX is set, add -static-libgcc.
	* configure: Rebuild.
gcc/:
	* Makefile.in (HOST_LIBS): Move higher in file.
	(LINKER, LINKER_FLAGS): If ENABLE_BUILD_WITH_CXX, but $(HOST_LIBS)
	is not empty, set to $(CC) and $(CFLAGS).

From-SVN: r154422