Always use AC_LANG_WERROR when checking for compiler warning flags

Many gcc-compatible compilers (icc, clang, etc.) verbosely ignore
gcc command line flags issuing a warning of some sort.  Catch these
warnings and turn them into errors when checking for support of
specific warning flags.
const'ify string literals

(librdf_query_virtuoso_init): const'ify seps

redland-virtuoso-test (main): const'ify query_cmd
Merge pull request #10 from hroptatyr/compiler-support

Intel C compiler (icc) support