mwaw: fix build failure with clang (work around libtool stupidity)

Clang warns about unused -I parameters, and the libtool configure check
for lt_cv_prog_compiler_pic_works_CXX mis-interprets that and declares
-fPIC is not supported.

Change-Id: I0802719142c5a96bc672be299d6e94a1b2d27001

external/libmwaw/ExternalProject_libmwaw.mk

@@ -36,7 +36,10 @@ $(call gb_ExternalProject_get_state_target,libmwaw,build) :
 			$(if $(ENABLE_DEBUG),--enable-debug,--disable-debug) \
 			$(if $(VERBOSE)$(verbose),--disable-silent-rules,--enable-silent-rules) \
 			--disable-werror \
-			CXXFLAGS="$(if $(SYSTEM_BOOST),$(BOOST_CPPFLAGS),-I$(call gb_UnpackedTarball_get_dir,boost) -I$(BUILDDIR)/config_$(gb_Side))" \
+			CXXFLAGS="$(if $(SYSTEM_BOOST),$(BOOST_CPPFLAGS),\
+				$(if $(COM_GCC_IS_CLANG),-Qunused-arguments) \
+				-I$(BUILDDIR)/config_$(gb_Side) \
+				-I$(call gb_UnpackedTarball_get_dir,boost))" \
 			$(if $(CROSS_COMPILING),--build=$(BUILD_PLATFORM) --host=$(HOST_PLATFORM)) \
 			$(if $(filter MACOSX,$(OS)),--prefix=/@.__________________________________________________OOO) \
 		&& (cd $(EXTERNAL_WORKDIR)/src/lib && \