odk: CustomTarget_build-examples hard-codes on "gcc" and "g++"

... so it fails if these aren't available; instead use the
C++ compiler of the LO build system.

Change-Id: I8a95d1865c481dc0a790d6fec6ff9fae8284d76d
Reviewed-on: https://gerrit.libreoffice.org/67078Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
Tested-by: Jenkins

odk/build-examples_common.mk

@@ -31,6 +31,7 @@ endif
 	$(foreach my_dir,$(2), \
 	    && (cd $(INSTDIR)/$(SDKDIRNAME)/examples/$(my_dir) \
 		&& printf 'yes\n' | LC_ALL=C make \
+			CC="$(CXX)" LINK="$(CXX)" LIB="$(CXX)" \
 		    $(if $(filter MACOSX,$(OS)), SHELL=$(ODK_BUILD_SHELL), )))) \
 	    >$(call gb_CustomTarget_get_workdir,$(1))/log 2>&1 \
 	|| (RET=$$$$? \