[build] Avoid using -ffunction-sections on some older versions of gcc

Some older versions of gcc issue a warning if -ffunction-sections is
used in combination with -g (gcc bug #18553).  Inhibit
-ffunction-sections when building with such a version of gcc.

Reported-by: zhengwei <zw111_2001@126.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/Makefile.housekeeping

@@ -450,14 +450,20 @@ endif
 
 # Enable per-item sections and section garbage collection.  Note that
 # some older versions of gcc support -fdata-sections but treat it as
-# implying -fno-common, which would break our build.
+# implying -fno-common, which would break our build.  Some other older
+# versions issue a spurious and uninhibitable warning if
+# -ffunction-sections is used with -g, which would also break our
+# build since we use -Werror.
 #
 ifeq ($(CCTYPE),gcc)
 DS_TEST		= $(ECHO) 'char x;' | \
 		  $(CC) -fdata-sections -S -x c - -o - 2>/dev/null | \
 		  grep -E '\.comm' > /dev/null
 DS_FLAGS	:= $(shell $(DS_TEST) && $(ECHO) '-fdata-sections')
-CFLAGS		+= -ffunction-sections $(DS_FLAGS)
+FS_TEST		= $(CC) -ffunction-sections -g -c -x c /dev/null \
+		  -o /dev/null 2>/dev/null
+FS_FLAGS	:= $(shell $(FS_TEST) && $(ECHO) '-ffunction-sections')
+CFLAGS		+= $(FS_FLAGS) $(DS_FLAGS)
 endif
 LDFLAGS		+= --gc-sections