$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.