- store the used compiler plus flags in the .%.dep files too and

  use this info to decide whether or not to rebuild something (if e.g. the
  filestamp of the prereq did not change but the cc or CFLAGS did).

  For files that we did not yet build we have no flags on record, so those
  did change inherently and we rebuild.

  Shouldn't be much slower than before.

Makerules

@@ -60,6 +60,8 @@ endif
 show_objs = $(subst ../,,$@)
 
 pur_disp_compile.c = echo "  "CC $(show_objs)
+pur_disp_compile.i = echo "  "CPP $(show_objs)
+pur_disp_compile.s = echo "  "CC-S $(show_objs)
 pur_disp_compile.u = echo "  "CC $(show_objs)
 pur_disp_compile.S = echo "  "AS $(show_objs)
 pur_disp_compile.m = $(pur_disp_compile.c)
@@ -70,6 +72,8 @@ pur_disp_ar        = echo "  "AR $(ARFLAGS) $@
 pur_disp_ld        = echo "  "LD $(1)
 
 sil_disp_compile.c = true
+sil_disp_compile.i = true
+sil_disp_compile.s = true
 sil_disp_compile.u = true
 sil_disp_compile.S = true
 sil_disp_compile.m = true
@@ -80,6 +84,8 @@ sil_disp_ar        = true
 sil_disp_ld        = true
 
 ver_disp_compile.c = echo $(cmd_compile.c)
+ver_disp_compile.i = echo $(cmd_compile.i)
+ver_disp_compile.s = echo $(cmd_compile.s)
 ver_disp_compile.u = echo $(cmd_compile.u)
 ver_disp_compile.S = echo $(cmd_compile.S)
 ver_disp_compile.m = echo $(cmd_compile.m)
@@ -90,6 +96,8 @@ ver_disp_ar        = echo $(cmd_ar)
 ver_disp_ld        =
 
 disp_compile.c = $($(DISP)_disp_compile.c)
+disp_compile.i = $($(DISP)_disp_compile.i)
+disp_compile.s = $($(DISP)_disp_compile.s)
 disp_compile.u = $($(DISP)_disp_compile.u)
 disp_compile.S = $($(DISP)_disp_compile.S)
 disp_compile.m = $($(DISP)_disp_compile.m)
@@ -99,22 +107,38 @@ disp_t_strip   = $($(DISP)_disp_t_strip)
 disp_ar        = $($(DISP)_disp_ar)
 disp_ld        = $($(DISP)_disp_ld)
 
-cmd_gen.dep = -MT $@ -MD -MF $(dir $@).$(notdir $@).dep
-
-cmd_compile.c = $(CC) -c $< -o $@ $(CFLAGS) $(ARCH_CFLAGS) $(CFLAGS-$(suffix $@)) $(filter-out $(CFLAGS-OMIT-$(notdir $<)),$(CFLAGS-$(notdir $(^D)))) $(CFLAGS-$(subst $(top_srcdir),,$(dir $<))) $(CFLAGS-$(notdir $<)) $(CFLAGS-$(notdir $@))  $(cmd_gen.dep)
-cmd_compile.u = $(CC) $^ $(DEPS-$(notdir $@)) -o $@ $(CFLAGS) $(CFLAGS-$(notdir $(^D))) $(CFLAGS-$(notdir $@)) $(cmd_gen.dep)
+CFLAGS_gen.dep = -MT $@ -MD -MF $(dir $@).$(notdir $@).dep
+
+# True if not identical. Neither order nor whitespace nor identical flags
+# matter.
+compare_flags = $(strip $(filter-out $(cmd_$(1)), $(cmd_$(@))) \
+			$(filter-out $(cmd_$(@)), $(cmd_$(1))))
+
+# Rebuild if the used CC or flags changed.
+# Previously used flags are stored in the corresponding .%.dep files
+maybe_exec = $(if $(strip $(compare_flags)), \
+		@set -e; \
+		$(disp_$(1)); \
+		$(cmd_$(1)); \
+		echo 'cmd_$@ := $(cmd_$1)' >> $(dir $@).$(notdir $@).dep)
+
+cmd_compile.c = $(CC) -c $< -o $@ $(CFLAGS) $(ARCH_CFLAGS) $(CFLAGS-$(suffix $@)) $(filter-out $(CFLAGS-OMIT-$(notdir $<)),$(CFLAGS-$(notdir $(^D)))) $(CFLAGS-$(subst $(top_srcdir),,$(dir $<))) $(CFLAGS-$(notdir $<)) $(CFLAGS-$(notdir $@))  $(CFLAGS_gen.dep)
+cmd_compile.i = $(cmd_compile.c:-c=-E -dD)
+cmd_compile.s = $(cmd_compile.c:-c=-S)
+cmd_compile.u = $(CC) $^ $(DEPS-$(notdir $@)) -o $@ $(CFLAGS) $(CFLAGS-$(notdir $(^D))) $(CFLAGS-$(notdir $@)) $(CFLAGS_gen.dep)
 cmd_compile.S = $(filter-out -std=gnu99, $(cmd_compile.c)) -D__ASSEMBLER__ $(ASFLAGS) $(ARCH_ASFLAGS) $(ASFLAGS-$(suffix $@)) $(ASFLAGS-$(notdir $<)) $(ASFLAGS-$(notdir $@))
 cmd_compile.m = $(cmd_compile.c) -DL_$(patsubst %$(suffix $(notdir $@)),%,$(notdir $@))
+
 cmd_compile-m = $(CC) $^ -c -o $@ $(CFLAGS) $(ARCH_CFLAGS) $(CFLAGS-$(suffix $@)) $(CFLAGS-$(notdir $(@D))) $(CFLAGS-$(notdir $@))
 cmd_strip     = $(STRIPTOOL) $(STRIP_FLAGS) $^
 cmd_t_strip   = $(STRIPTOOL) $(STRIP_FLAGS) $@
 cmd_ar        = $(AR) $(ARFLAGS) $@ $^
 
-compile.c = @$(disp_compile.c) ; $(cmd_compile.c)
-compile.i = $(cmd_compile.c:-c=-E -dD)
-compile.s = $(cmd_compile.c:-c=-S)
-compile.S = @$(disp_compile.S) ; $(cmd_compile.S)
-compile.m = @$(disp_compile.m) ; $(cmd_compile.m)
+compile.c = @$(call maybe_exec,compile.c)
+compile.i =  $(call maybe_exec,compile.i)
+compile.s =  $(call maybe_exec,compile.s)
+compile.S = @$(call maybe_exec,compile.S)
+compile.m = @$(call maybe_exec,compile.m)
 compile-m = @$(disp_compile-m) ; $(cmd_compile-m)
 do_strip  = @$(disp_strip)     ; $(cmd_strip)
 do_t_strip= @$(disp_t_strip)   ; $(cmd_t_strip)
@@ -180,19 +204,19 @@ endef
 CFLAGS-.os+=$(PICFLAG)
 CFLAGS-.oS+=$(PICFLAG) -DSHARED
 
-%.o:  %.c ; $(compile.c)
-%.os: %.c ; $(compile.c)
-%.oS: %.c ; $(compile.c)
-%.o:  %.S ; $(compile.S)
-%.os: %.S ; $(compile.S)
-%.oS: %.S ; $(compile.S)
-%.o:  %.s ; $(compile.S)
-%.os: %.s ; $(compile.S)
-%.oS: %.s ; $(compile.S)
-%.i:  %.c ; $(compile.i)
-%.i:  %.S ; $(compile.i)
-%.s:  %.c ; $(compile.s)
-%.s:  %.S ; $(compile.s)
+%.o:  %.c FORCE ; $(compile.c)
+%.os: %.c FORCE ; $(compile.c)
+%.oS: %.c FORCE ; $(compile.c)
+%.o:  %.S FORCE ; $(compile.S)
+%.os: %.S FORCE ; $(compile.S)
+%.oS: %.S FORCE ; $(compile.S)
+%.o:  %.s FORCE ; $(compile.S)
+%.os: %.s FORCE ; $(compile.S)
+%.oS: %.s FORCE ; $(compile.S)
+%.i:  %.c FORCE ; $(compile.i)
+%.i:  %.S FORCE ; $(compile.i)
+%.s:  %.c FORCE ; $(compile.s)
+%.s:  %.S FORCE ; $(compile.s)
 
 $(top_builddir)lib/interp.c: | $(sub_headers)
 	$(Q)$(INSTALL) -d $(dir $@)
@@ -306,7 +330,9 @@ ifdef .depends.dep
 -include $(.depends.dep)
 endif
 
-.PHONY: dummy create
+.PHONY: dummy FORCE
+FORCE:
+
 clean: objclean-y headers_clean-y
 realclean: clean
 	$(Q)$(RM) $(.depends.dep)