CMakeLists.txt: Allow overriding PYTHON_INSTALL_DIR

Useful for testing. The dnf-plugins-core RPM installs Python modules to
/usr/lib/python3.XX/site-packages whereas a `make install` will by
default install files to /usr/local/lib/python3.XX/site-packages.

CMakeLists.txt

@@ -22,7 +22,9 @@ ELSE ()
     MESSAGE (FATAL_ERROR "Invalid PYTHON_DESIRED value: " ${PYTHON_DESIRED})
 ENDIF()
 
-EXECUTE_PROCESS(COMMAND ${PYTHON_EXECUTABLE} -c "from sys import stdout; from sysconfig import get_path; stdout.write(get_path('purelib'))" OUTPUT_VARIABLE PYTHON_INSTALL_DIR)
+IF (not PYTHON_INSTALL_DIR)
+    EXECUTE_PROCESS(COMMAND ${PYTHON_EXECUTABLE} -c "from sys import stdout; from sysconfig import get_path; stdout.write(get_path('purelib'))" OUTPUT_VARIABLE PYTHON_INSTALL_DIR)
+ENDIF()
 MESSAGE(STATUS "Python install dir is ${PYTHON_INSTALL_DIR}")
 
 SET (SYSCONFDIR /etc)