iwyu: add language standard defines to flags

for some reason they are not set and cause parse warnings

Change-Id: I1bbc14da8cd7f4cbde8e59934b6ace932245e2a1
Reviewed-on: https://gerrit.libreoffice.org/39093Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>

bin/gen-iwyu-dummy-lib

@@ -47,7 +47,7 @@ mkdir -p "$iwyu_DIR"
 
     echo '$(eval $(call gb_StaticLibrary_StaticLibrary,iwyudummy))'
     # clang will "compile" headers to .gch by default
-    echo '$(eval $(call gb_StaticLibrary_add_cxxflags,iwyudummy,-x c++ -Wno-unused-macros -Wno-unused-const-variable))'
+    echo '$(eval $(call gb_StaticLibrary_add_cxxflags,iwyudummy,-x c++ -D__cplusplus=201402L -D__STDC_VERSION__=201112L -Wno-unused-macros -Wno-unused-const-variable))'
     echo '$(eval $(call gb_StaticLibrary_use_custom_headers,iwyudummy,officecfg/registry))'
     echo '$(eval $(call gb_StaticLibrary_use_sdk_api,iwyudummy))'
     echo '$(eval $(call gb_StaticLibrary_use_externals,iwyudummy,\'