cmake: Allow PYTHON_SITE_PACKAGES_INSTALL_DIR set extern (#461)

The variable PYTHON_SITE_PACKAGES_INSTALL_DIR is kind of
hardcoded in the CMakeLists.txt to .../dist-packages. This
is not true for all linux distros. dist-packages comes
from debian and debian like distros. Python core uses
site-packages.

We use an unintrusive way and allow the variable to be
passed from cmake, so you at least have the option to
set a different install location (i.e. site-packages)
from extern.

Signed-off-by: Matthias Schoepfer <matthias.schoepfer@googlemail.com>

python_orocos_kdl/CMakeLists.txt

@@ -28,7 +28,7 @@ set(PYBIND11_PYTHON_VERSION ${PYTHON_VERSION} CACHE STRING "Python version used
 find_package(Python ${PYTHON_VERSION} COMPONENTS Interpreter Development REQUIRED)
 # get_python_lib in python3 produces path which isn't in sys.path: https://bugs.launchpad.net/ubuntu/+source/python3-stdlib-extensions/+bug/1832215
 # execute_process(COMMAND ${PYTHON_EXECUTABLE} -c "from distutils.sysconfig import get_python_lib; print(get_python_lib(plat_specific=True, prefix=''))" OUTPUT_VARIABLE PYTHON_SITE_PACKAGES OUTPUT_STRIP_TRAILING_WHITESPACE)
-set(PYTHON_SITE_PACKAGES_INSTALL_DIR "${CMAKE_INSTALL_PREFIX}/lib/python${Python_VERSION_MAJOR}.${Python_VERSION_MINOR}/dist-packages")  # This might be overridden below if built with catkin.
+set(PYTHON_SITE_PACKAGES_INSTALL_DIR "${CMAKE_INSTALL_PREFIX}/lib/python${Python_VERSION_MAJOR}.${Python_VERSION_MINOR}/dist-packages" CACHE STRING "Install location of the python package")  # This might be overridden below if built with catkin.
 set(LIBRARY_NAME "PyKDL")
 
 # catkin-specific configuration (optional)