Find Python Headers on CentOS

Modifies the CMake configuration so that the Python header is found
on CentOS. CentOS includes an older version of CMake, which uses the
deprecated PYTHON_INCLUDE_PATH variable instead of PYTHON_INCLUDE_DIRS.

Changes:

 * CMakeLists.txt: Add PYTHON_INCLUDE_PATH to the header search path.
 * combine_perfReses_dialog.cpp: Remove conditional include for OS X,
   since CMake takes care of finding the header for us.