builds externals without werror

I get a warning under MacOSX of

argument unused during compilation -I . -Werror,-Wunused-command-line-argument

Change-Id: I0ae48783eefa52a829471e51a94c79d235dc2f38
Reviewed-on: https://gerrit.libreoffice.org/39988Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

external/cppunit/ExternalProject_cppunit.mk

@@ -47,6 +47,7 @@ $(call gb_ExternalProject_get_state_target,cppunit,build) :
 			--disable-doxygen \
 			--disable-html-docs \
 			--disable-latex-docs \
+			--disable-werror \
 			$(if $(CROSS_COMPILING),--build=$(BUILD_PLATFORM) --host=$(HOST_PLATFORM)) \
 			$(if $(filter MACOSX,$(OS)),--prefix=/@.__________________________________________________NONE) \
 			$(if $(filter WNT,$(OS)),LDFLAGS="-Wl$(COMMA)--enable-runtime-pseudo-reloc-v2") \