build: add centroiding C++ support

Centroiding source is in C++. This CL removes -W flags only for building
C files from CFLAGS and also prevents typedef existing C++ keyword
wchar_t.

BUG=b:120961468
BRANCH=none
TEST=make buildall

Change-Id: Ifb8793a8e7e69b26a742b7dbf70289747a0ee7b3
Signed-off-by: Rong Chang <rongchang@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1372874
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Reviewed-by: Tai-Hsu Lin <sheckylin@chromium.org>

Makefile.rules

@@ -78,7 +78,7 @@ cmd_fuzz_exe = $(CXX) $^ $(HOST_TEST_LDFLAGS) $(LDFLAGS_EXTRA) -o $@
 cmd_run_fuzz = build/host/$*/$*.exe -seed=1 -runs=1 $(silent) \
 	$(silent_err) || (echo "Test $* failed!" && false)
 cmd_exe = $(CC) $(ro-objs) $(HOST_TEST_LDFLAGS) -o $@
-cmd_c_to_o = $(CC) $(CFLAGS) -MMD -MP -MF $@.d -c $< -o $(@D)/$(@F)
+cmd_c_to_o = $(CC) $(C_WARN) $(CFLAGS) -MMD -MP -MF $@.d -c $< -o $(@D)/$(@F)
 cmd_cxx_to_o = $(CXX) -std=c++11 $(CFLAGS) $(CXXFLAGS) -MMD -MP -MF $@.d -c $< \
 		      -o $(@D)/$(@F)
 cmd_c_to_build = $(BUILDCC) $(BUILD_CFLAGS) \

Makefile.toolchain

@@ -92,7 +92,7 @@ ifneq ($(BOARD),host)
 CPPFLAGS+=-ffreestanding -fno-builtin -nostdinc -nostdlib
 CPPFLAGS+=-Ibuiltin/
 endif
-CFLAGS=$(CPPFLAGS) $(CFLAGS_CPU) $(CFLAGS_DEBUG) $(CFLAGS_WARN) $(CFLAGS_y)
+CFLAGS=$(CPPFLAGS) $(CFLAGS_CPU) $(CFLAGS_DEBUG) $(COMMON_WARN) $(CFLAGS_y)
 CFLAGS+= -ffunction-sections -fshort-wchar
 ifneq ($(cc-name),clang)
 # TODO(chromium:854924): LLVM support for this flag is coming soon.

builtin/stddef.h

@@ -25,7 +25,10 @@ typedef __SIZE_TYPE__ ssize_t;
 #ifndef __WCHAR_TYPE__
 #define __WCHAR_TYPE__ int
 #endif
+
+#ifndef __cplusplus
 typedef __WCHAR_TYPE__ wchar_t;
+#endif
 
 /* This macro definition is duplicated in compile_time_macros.h. It still needs
  * to be defined here to support code that expects offsetof to be defined in the