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 and char pointers that needn't be changed

roqet (main): const'ify feature_value_type
Merge pull request #3 from hroptatyr/compiler-support

Intel C compiler (icc) support