Fix mosquitto_pub -l if compiled using cmake.

Since dde005ef

 mosquito_pub is throwing error
that 'threading support has not been compiled' when compiled using cmake.
It looks like WITH_THREADING flag is not set at top level Makefile and used
only in lib/ directory, so library is correctly compiled with threading.
But for client this flag is undefined, so it gives error on '-l' option.

This commit moves part related to WITH_THREADING flag out of lib/CMakeLists.txt
to top levele CMakeLists.txt, so it could be accessible to all subdirectories.

Signed-off-by: Marek Wodzinski <majek@w7i.pl>