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

librdfa (start_element): const'ify umap_key

librdfa (rdfa_complete_object_literal_triples): const'ifty current_object_literal

rapper (print_graph): const'ify label
Cosmetics, make make V=0 look good

Using automake's AM_V_GEN trickery to hide the lex and bison calls.
This will also hide compiler warnings erupting from generated code.
Merge pull request #14 from hroptatyr/compiler-support

Intel C compiler (icc) support