* tools/CMakeLists.txt: Fix missing POPT_INCLUDE_DIRS variable.

Building master on Mac OS X with port fails on a missing include file <popt.h> in tools/common.h. The file is installed and found by cmake, but the correct path (/opt/local/include) is stored in POPT_INCLUDE_DIR instead of POPT_INCLUDE_DIRS. This patch fixes that.
I suppose this problem can only occur when popt.h is not installed in the target installation folder.
Lieven