build: fix libcap configure flag handling

build: fix libcap configure flag handling

* m4/jm-macros.m4 (coreutils_MACROS): The code to handle configure-time
enabling or disabling of libcap support was broken.  It would treat any
libcap configure option as --disable-libcap because it doesn't check
$enableval at all.  This change makes sure we do the sane thing:
  --disable-libcap -> disable and don't run any tests
  --enable-libcap -> run tests and fail if not found
  default -> run tests and warn if not found