firebird: fix CXXFLAGS

In 3.0.4, the horrible makefiles override the CXXFLAGS variable:

firebird/builds/posix/prefix.linux_generic:CXXFLAGS=-std=gnu++03

Work around that by passing LO's customised CXXFLAGS as a command-line
parameter to make, which in turn overrides the definition in the
makefile.

Other platforms, in particular Darwin, apparently extend CXXFLAGS
instead of overwriting.

To nobody's surprise, gnu++03 doesn't even build with clang because ICU
headers use C++11 features (although strangely gcc doesn't complain).

Change-Id: If3b26482a4f4bf284057e261677cd7182656154f
Reviewed-on: https://gerrit.libreoffice.org/67255
Tested-by: Jenkins
Reviewed-by: Michael Stahl <Michael.Stahl@cib.de>

external/firebird/ExternalProject_firebird.mk

@@ -97,9 +97,17 @@ $(call gb_ExternalProject_get_state_target,firebird,build):
 							'<' 101200)), \
 					ac_cv_func_clock_gettime=no)) \
 		&& if [ -n "$${FB_CPU_ARG}" ]; then \
-			   $(MAKE_PRE) $(MAKE) $(if $(ENABLE_DEBUG),Debug) $(INVOKE_FPA) SHELL='$(SHELL)' LIBO_TUNNEL_LIBRARY_PATH='$(subst ','\'',$(subst $$,$$$$,$(call gb_Helper_extend_ld_path,$(call gb_UnpackedTarball_get_dir,icu)/source/lib)))' $(MAKE_POST); \
+				$(MAKE_PRE) $(MAKE) \
+					$(if $(filter LINUX,$(OS)),CXXFLAGS="$$CXXFLAGS -std=gnu++11") \
+					$(if $(ENABLE_DEBUG),Debug) $(INVOKE_FPA) SHELL='$(SHELL)' \
+					LIBO_TUNNEL_LIBRARY_PATH='$(subst ','\'',$(subst $$,$$$$,$(call gb_Helper_extend_ld_path,$(call gb_UnpackedTarball_get_dir,icu)/source/lib)))' \
+				$(MAKE_POST); \
 			else \
-			   $(MAKE_PRE) $(MAKE) $(if $(ENABLE_DEBUG),Debug) SHELL='$(SHELL)' LIBO_TUNNEL_LIBRARY_PATH='$(subst ','\'',$(subst $$,$$$$,$(call gb_Helper_extend_ld_path,$(call gb_UnpackedTarball_get_dir,icu)/source/lib)))' $(MAKE_POST); \
+				$(MAKE_PRE) $(MAKE) \
+					$(if $(filter LINUX,$(OS)),CXXFLAGS="$$CXXFLAGS -std=gnu++11") \
+					$(if $(ENABLE_DEBUG),Debug) SHELL='$(SHELL)' \
+					LIBO_TUNNEL_LIBRARY_PATH='$(subst ','\'',$(subst $$,$$$$,$(call gb_Helper_extend_ld_path,$(call gb_UnpackedTarball_get_dir,icu)/source/lib)))' \
+				$(MAKE_POST); \
 			fi \
 	)
 # vim: set noet sw=4 ts=4: