Don't recursively evaluate CFLAGS variables.

The $(cc-option) macro was being executed multiple times for each
compiler invocation.  That's a waste of time.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

Makefile

@@ -25,7 +25,7 @@ SRC32SEG=util.c output.c pci.c pcibios.c apm.c stacks.c
 cc-option=$(shell if test -z "`$(1) $(2) -S -o /dev/null -xc /dev/null 2>&1`" \
     ; then echo "$(2)"; else echo "$(3)"; fi ;)
 
-COMMONCFLAGS = -I$(OUT) -Os -MD -g \
+COMMONCFLAGS := -I$(OUT) -Os -MD -g \
     -Wall -Wno-strict-aliasing -Wold-style-definition \
     $(call cc-option,$(CC),-Wtype-limits,) \
     -m32 -march=i386 -mregparm=3 -mpreferred-stack-boundary=2 \
@@ -36,14 +36,14 @@ COMMONCFLAGS += $(call cc-option,$(CC),-nopie,)
 COMMONCFLAGS += $(call cc-option,$(CC),-fno-stack-protector,)
 COMMONCFLAGS += $(call cc-option,$(CC),-fno-stack-protector-all,)
 
-CFLAGS32FLAT = $(COMMONCFLAGS) -DMODE16=0 -DMODESEGMENT=0 -fomit-frame-pointer
-CFLAGSSEG = $(COMMONCFLAGS) -DMODESEGMENT=1 -fno-defer-pop \
+CFLAGS32FLAT := $(COMMONCFLAGS) -DMODE16=0 -DMODESEGMENT=0 -fomit-frame-pointer
+CFLAGSSEG := $(COMMONCFLAGS) -DMODESEGMENT=1 -fno-defer-pop \
     $(call cc-option,$(CC),-fno-jump-tables,-DMANUAL_NO_JUMP_TABLE) \
     $(call cc-option,$(CC),-fno-tree-switch-conversion,)
-CFLAGS32SEG = $(CFLAGSSEG) -DMODE16=0 -fomit-frame-pointer
-CFLAGS16INC = $(CFLAGSSEG) -DMODE16=1 -Wa,src/code16gcc.s \
+CFLAGS32SEG := $(CFLAGSSEG) -DMODE16=0 -fomit-frame-pointer
+CFLAGS16INC := $(CFLAGSSEG) -DMODE16=1 -Wa,src/code16gcc.s \
     $(call cc-option,$(CC),--param large-stack-frame=4,-fno-inline)
-CFLAGS16 = $(CFLAGS16INC) -fomit-frame-pointer
+CFLAGS16 := $(CFLAGS16INC) -fomit-frame-pointer
 
 # Run with "make V=1" to see the actual compile commands
 ifdef V