avoid C++11 narrowing conversion error with clang

/home/tinderbox/clang-master-build/solver/unxlngi6/inc/external/boost/uuid/seed_rng.hpp:143:61:
error: non-constant-expression cannot be narrowed from type 'int' to 'unsigned int' in initializer list [-Wc++11-narrowing]
            unsigned int rn[] = { std::rand(), std::rand(), std::rand() };

Change-Id: Iea9808492cd3fbcf078e61bbc184bdc434e7cd8a

libcmis/StaticLibrary_cmis.mk

@@ -11,6 +11,14 @@ $(eval $(call gb_StaticLibrary_StaticLibrary,cmislib))
 
 $(eval $(call gb_StaticLibrary_set_warnings_not_errors,cmislib))
 
+ifeq ($(COM_GCC_IS_CLANG),TRUE)
+# Avoid narrowing conversion error (even though the option is technically a warning)
+# caused by boost.
+$(eval $(call gb_StaticLibrary_add_cxxflags,cmislib,\
+    -Wno-error=c++11-narrowing \
+))
+endif
+
 $(eval $(call gb_StaticLibrary_use_unpacked,cmislib,cmis))
 
 $(eval $(call gb_StaticLibrary_use_package,cmislib,libcmis_inc))