c-index-test/Makefile: Use -isystem instead of -I on $(LIBXML2_INC) with -Wdocumentation.

-Wdocumentation won't seek -isystem. LIBXML2's headers in a certain distro might be incompatible to -Wdocumentation.

FIXME: Could autoconf detect clang or availability of -isystem?
c-index-test/CMakeLists.txt: Suggest -isystem to include libxml2 with include_directories(SYSTEM).