[build] Use positive-form tests when checking for supported warnings

Some versions of gcc seem to silently accept an attempt to disable an
unrecognised warning (e.g. via -Wno-stringop-truncation) but will then
report the unrecognised warning if any other error occurs during the
build, resulting in a potentially misleading error message.

Avoid this potential confusion by using the positive-form tests in
order to determine the workaround CFLAGS.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/Makefile.housekeeping

 # Inhibit this.
 #
 ifeq ($(CCTYPE),gcc)
-WNA_TEST = $(CC) -Wno-address -x c -c /dev/null -o /dev/null >/dev/null 2>&1
+WNA_TEST = $(CC) -Waddress -x c -c /dev/null -o /dev/null >/dev/null 2>&1
 WNA_FLAGS := $(shell $(WNA_TEST) && $(ECHO) '-Wno-address')
 WORKAROUND_CFLAGS += $(WNA_FLAGS)
 
 # gcc 8.0 generates warnings for certain suspect string operations. Our
 # sources have been vetted for correct usage.  Turn off these warnings.
 #
-WNST_TEST = $(CC) -Wno-stringop-truncation -x c -c /dev/null -o /dev/null \
+WNST_TEST = $(CC) -Wstringop-truncation -x c -c /dev/null -o /dev/null \
 		  >/dev/null 2>&1
 WNST_FLAGS := $(shell $(WNST_TEST) && $(ECHO) '-Wno-stringop-truncation')
 WORKAROUND_CFLAGS += $(WNST_FLAGS)