Remove the need to set -I flags in Makefile.libwheel

This avoids unneeded pollution of CPPFLAGS by Makefile.libwheel, making it
friendlier for other inclusion in other Makefiles.
wmem: Use proper name for w_lmem_cleanup() function

The "w_lmem" macro in wheel.h refers to w_lmem_cleanup(), but the
function was incorrectly implemented with name w_mem_cleanup().