build: Use NACL_{INC,LIB}_DIR set by configure

The environment variables $NACL_INC_DIR/$NACL_LIB_DIR might not always
be set, but the configure script will attempt to detect an existing NaCl
installation if they aren't. Thus use the
$CONFIG_NACL_INC_DIR/$CONFIG_NACL_LIB_DIR variables as emitted by the
configure script for the build.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>