optionrom: build with discrete CPP and AS steps

Build option ROM .S files with separate preprocessor and
assembler steps because the C compiler could be unsuitable.

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>

configure

@@ -264,6 +264,8 @@ else
 fi
 
 ar="${AR-${cross_prefix}ar}"
+as="${AS-${cross_prefix}as}"
+cpp="${CPP-$cc -E}"
 objcopy="${OBJCOPY-${cross_prefix}objcopy}"
 ld="${LD-${cross_prefix}ld}"
 libtool="${LIBTOOL-${cross_prefix}libtool}"
@@ -3726,6 +3728,8 @@ echo "CC_I386=$cc_i386" >> $config_host_mak
 echo "HOST_CC=$host_cc" >> $config_host_mak
 echo "OBJCC=$objcc" >> $config_host_mak
 echo "AR=$ar" >> $config_host_mak
+echo "AS=$as" >> $config_host_mak
+echo "CPP=$cpp" >> $config_host_mak
 echo "OBJCOPY=$objcopy" >> $config_host_mak
 echo "LD=$ld" >> $config_host_mak
 echo "WINDRES=$windres" >> $config_host_mak
@@ -4277,9 +4281,10 @@ for rom in seabios vgabios ; do
     config_mak=roms/$rom/config.mak
     echo "# Automatically generated by configure - do not modify" > $config_mak
     echo "SRC_PATH=$source_path/roms/$rom" >> $config_mak
+    echo "AS=$as" >> $config_mak
     echo "CC=$cc" >> $config_mak
     echo "BCC=bcc" >> $config_mak
-    echo "CPP=${cross_prefix}cpp" >> $config_mak
+    echo "CPP=$cpp" >> $config_mak
     echo "OBJCOPY=objcopy" >> $config_mak
     echo "IASL=iasl" >> $config_mak
     echo "LD=$ld" >> $config_mak

rules.mak

@@ -28,8 +28,11 @@ else
 	$(call quiet-command,$(LIBTOOL) --mode=compile --quiet --tag=CC $(CC) $(QEMU_INCLUDES) $(QEMU_CFLAGS) $(QEMU_DGFLAGS) $(CFLAGS) -c -o $@ $<,"  lt CC $@")
 endif
 
-%.o: %.S
-	$(call quiet-command,$(CC) $(QEMU_INCLUDES) $(QEMU_CFLAGS) $(QEMU_DGFLAGS) $(CFLAGS) -c -o $@ $<,"  AS    $(TARGET_DIR)$@")
+%.asm: %.S
+	$(call quiet-command,$(CPP) $(QEMU_INCLUDES) $(QEMU_CFLAGS) $(QEMU_DGFLAGS) $(CFLAGS) -o $@ $<,"  CPP   $(TARGET_DIR)$@")
+
+%.o: %.asm
+	$(call quiet-command,$(AS) $(ASFLAGS) -o $@ $<,"  AS    $(TARGET_DIR)$@")
 
 %.o: %.m
 	$(call quiet-command,$(OBJCC) $(QEMU_INCLUDES) $(QEMU_CFLAGS) $(QEMU_DGFLAGS) $(CFLAGS) -c -o $@ $<,"  OBJC  $(TARGET_DIR)$@")