[build] Avoid spurious address comparison warnings in gcc 4.6

A construction such as "assert ( ptr != NULL )" seems to trigger a
false positive warning in gcc 4.6 if the value of "ptr" is known at
compile-time to be non-NULL.  Use -Wno-address to inhibit this
warning.

Reported-by: Ralph Giles <giles@thaumas.net>
Tested-by: Ralph Giles <giles@thaumas.net>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/Makefile.housekeeping

 WORKAROUND_CFLAGS += $(CFI_FLAGS)
 endif
 
+# gcc 4.6 generates spurious warnings if -Waddress is in force.
+# Inhibit this.
+#
+ifeq ($(CCTYPE),gcc)
+WNA_TEST = $(CC) -Wno-address -x c -c /dev/null -o /dev/null >/dev/null 2>&1
+WNA_FLAGS := $(shell $(WNA_TEST) && $(ECHO) '-Wno-address')
+WORKAROUND_CFLAGS += $(WNA_FLAGS)
+endif
+
 # Some versions of gas choke on division operators, treating them as
 # comment markers.  Specifying --divide will work around this problem,
 # but isn't available on older gas versions.