clover: Add --with-clang-libdir option and verify CLANG_RESOURCE_DIR
$CLANG_RESOURCE_DIR is the directory that contains all resources needed by clang to compile programs. When clover uses clang to compile kernels it needs to specify a resource dir, so that clang can find its internal headers (e.g. stddef.h). clang defines $CLANG_RESOURCE_DIR as $CLANG_LIBDIR/clang/$CLANG_VERSION This patch adds the --with-clang-libdir option in order to accommodate clang intalls to non-standard locations, and it also adds a check to the configure script to verify that $CLANG_RESOURCE_DIR/include contains the necessary header files.