package/jimtcl: fix fallout after no-C++ fixups

package/jimtcl: fix fallout after no-C++ fixups

Commit 4cd1ab158 (core: alternate solution to disable C++) made use of a
non-existent 'no' binary when C++ is not available in the toolchain.

However, some packages, like jimtcl, really want to find the binary that
$CXX contains.

Revert jimtcl to use 'false' instead of 'no'.

Fixes:
    http://autobuild.buildroot.org/results/54f/54f3df03551fbdf293d33dc1e3f08005faa15321/

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>