clang, as opposed to gcc, does not terminate with a non-zero return code

clang, as opposed to gcc, does not terminate with a non-zero return code
in case an unrecognised command line option is encountered. configure.ac
however was assuming just that which led to compile time warnings later on.
Add -Werror to the configure bits to make clang behave like gcc in this
regard. Fixes BZ #348565.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@15323