tools build: Add CFLAGS_REMOVE_* support

Adding support to remove options from final CFLAGS for both object file
and build target. It's now possible to remove CFLAGS options like:

CFLAGS_REMOVE_krava.o += -Wstrict-prototypes

Committer notes:

This comes from the kernel's kbuild infrastructure, the subset that is
supported in tools/ is being documented at tools/build/Documentation/Build.txt.

Signed-off-by: Jiri Olsa <[e-mail]>
Tested-by: Stephane Eranian <[e-mail]>
Cc: Andi Kleen <[e-mail]>
Cc: David Ahern <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: William Cohen <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/build/Build.include

···
 # - per target C flags
 # - per object C flags
 # - BUILD_STR macro to allow '-D"$(variable)"' constructs
-c_flags = -Wp,-MD,$(depfile),-MT,$@ $(CFLAGS) -D"BUILD_STR(s)=\#s" $(CFLAGS_$(basetarget).o) $(CFLAGS_$(obj))
+c_flags_1 = -Wp,-MD,$(depfile),-MT,$@ $(CFLAGS) -D"BUILD_STR(s)=\#s" $(CFLAGS_$(basetarget).o) $(CFLAGS_$(obj))
+c_flags_2 = $(filter-out $(CFLAGS_REMOVE_$(basetarget).o), $(c_flags_1))
+c_flags   = $(filter-out $(CFLAGS_REMOVE_$(obj)), $(c_flags_2))
 cxx_flags = -Wp,-MD,$(depfile),-MT,$@ $(CXXFLAGS) -D"BUILD_STR(s)=\#s" $(CXXFLAGS_$(basetarget).o) $(CXXFLAGS_$(obj))
 
 ###

tools/build/Documentation/Build.txt

···
 
 It's possible to alter the standard object C flags in the following way:
 
-  CFLAGS_perf.o += '...' - alters CFLAGS for perf.o object
-  CFLAGS_gtk += '...'    - alters CFLAGS for gtk build object
+  CFLAGS_perf.o        += '...'  - adds CFLAGS for perf.o object
+  CFLAGS_gtk           += '...'  - adds CFLAGS for gtk build object
+  CFLAGS_REMOVE_perf.o += '...'  - removes CFLAGS for perf.o object
+  CFLAGS_REMOVE_gtk    += '...'  - removes CFLAGS for gtk build object
 
 This C flags changes has the scope of the Build makefile they are defined in.