Bug 1271829 - Don't clobber XCFLAGS building nss. r=glandium

Previously, this clobbered the XCFLAGS set by MOZ_FOLD_LIBS_FLAGS.
Combine them instead so we can pass multiple options through
this override mechanism.

This is a behaviour change building with --enable-llvm-hacks.

MozReview-Commit-ID: 4WP0asnc7Z6

config/external/nss/Makefile.in

@@ -244,7 +244,7 @@ ifdef MOZ_NO_WLZDEFS
 DEFAULT_GMAKE_FLAGS += ZDEFS_FLAG=
 endif
 ifdef MOZ_CFLAGS_NSS
-DEFAULT_GMAKE_FLAGS += XCFLAGS='$(filter-out -W%,$(CFLAGS))'
+XCFLAGS += $(filter-out -W%,$(CFLAGS))
 DEFAULT_GMAKE_FLAGS += DARWIN_DYLIB_VERSIONS='-compatibility_version 1 -current_version 1 $(LDFLAGS)'
 endif
 ifeq (1_1,$(CLANG_CL)_$(MOZ_ASAN))
@@ -267,9 +267,12 @@ DEFAULT_GMAKE_FLAGS += MAKE_OBJDIR='$$(INSTALL) -D $$(OBJDIR)'
 DEFAULT_GMAKE_FLAGS += TARGETS='$$(LIBRARY) $$(SHARED_LIBRARY) $$(PROGRAM)'
 
 ifdef MOZ_FOLD_LIBS_FLAGS
-DEFAULT_GMAKE_FLAGS += XCFLAGS='$(MOZ_FOLD_LIBS_FLAGS)'
+XCFLAGS += $(MOZ_FOLD_LIBS_FLAGS)
 endif
 
+# Export accumulated XCFLAGS to modify nss defaults.
+DEFAULT_GMAKE_FLAGS += XCFLAGS='$(XCFLAGS)'
+
 NSS_SRCDIR = $(topsrcdir)
 
 NSS_DIRS =