pylibfdt/Makefile.pylibfdt: use project's flags to compile the extension

Seems the project's CFLAGS are not used when compiling the python
extension's C code via the setup.py script. Some default flags are used
instead. Thus pass the CFLAGS explicitly. Unfortunately the SWIG generated
code is not clean and requires -Wno-error override to compile successfully,
because -Werror is used in the project's flags.

Signed-off-by: Dan Horák <dan@danny.cz>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
diff --git a/pylibfdt/Makefile.pylibfdt b/pylibfdt/Makefile.pylibfdt
index 73c8b76..d20659c 100644
--- a/pylibfdt/Makefile.pylibfdt
+++ b/pylibfdt/Makefile.pylibfdt
@@ -17,7 +17,7 @@
 
 $(PYMODULE): $(PYLIBFDT_srcs) $(LIBFDT_archive) $(SETUP)
 	@$(VECHO) PYMOD $@
-	$(PYTHON) $(SETUP) $(SETUPFLAGS) build_ext
+	CFLAGS="$(CFLAGS) -Wno-error" $(PYTHON) $(SETUP) $(SETUPFLAGS) build_ext
 
 install_pylibfdt: $(PYMODULE)
 	@$(VECHO) INSTALL-PYLIB