optionrom: fix detection of -Wa,-32

The cc-option macro runs $(CC) in -S mode (generate assembly) to avoid a
pointless run of the assembler.  However, this does not work when you want
to detect support for cc->as option passthrough.  clang ignores -Wa unless
-c is provided, and exits successfully even if the -Wa,-32 option is not
supported.

Reported-by: Stefan Hajnoczi <stefanha@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1469043409-14033-1-git-send-email-pbonzini@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

pc-bios/optionrom/Makefile

@@ -30,7 +30,7 @@ QEMU_INCLUDES += -I$(SRC_PATH)
 
 Wa = -Wa,
 ASFLAGS += -32
-QEMU_CFLAGS += $(call cc-option, $(QEMU_CFLAGS), $(Wa)-32)
+QEMU_CFLAGS += $(call cc-c-option, $(QEMU_CFLAGS), $(Wa)-32)
 
 build-all: multiboot.bin linuxboot.bin linuxboot_dma.bin kvmvapic.bin
 

rules.mak

@@ -113,6 +113,8 @@ quiet-command = $(if $(V),$1,$(if $(2),@echo $2 && $1, @$1))
 
 cc-option = $(if $(shell $(CC) $1 $2 -S -o /dev/null -xc /dev/null \
               >/dev/null 2>&1 && echo OK), $2, $3)
+cc-c-option = $(if $(shell $(CC) $1 $2 -c -o /dev/null -xc /dev/null \
+                >/dev/null 2>&1 && echo OK), $2, $3)
 
 VPATH_SUFFIXES = %.c %.h %.S %.cc %.cpp %.m %.mak %.texi %.sh %.rc
 set-vpath = $(if $1,$(foreach PATTERN,$(VPATH_SUFFIXES),$(eval vpath $(PATTERN) $1)))