Python: Assume /usr/local prefix on Debian

If we're using the fallback path detection method on Debian, we
shouldn't compute the install path relative to `${CMAKE_INSTALL_PREFIX}`
since it _isn't_. So, we assume it's relative to `/usr/local` instead.

Author: ferdnyc

src/bindings/python/CMakeLists.txt

@@ -65,29 +65,39 @@ if (PYTHONLIBS_FOUND AND PYTHONINTERP_FOUND)
 	target_link_libraries(${SWIG_MODULE_pyopenshot_REAL_NAME}
 	                      ${PYTHON_LIBRARIES} openshot)
 
-    ### Check if the following Debian-friendly python module path exists
-	SET(PYTHON_MODULE_PATH "${CMAKE_INSTALL_PREFIX}/lib/python${PYTHON_VERSION_MAJOR}.${PYTHON_VERSION_MINOR}/site-packages")
-	if (NOT EXISTS ${PYTHON_MODULE_PATH})
-
-		### Check if another Debian-friendly python module path exists
-		SET(PYTHON_MODULE_PATH "${CMAKE_INSTALL_PREFIX}/lib/python${PYTHON_VERSION_MAJOR}.${PYTHON_VERSION_MINOR}/dist-packages")
-		if (NOT EXISTS ${PYTHON_MODULE_PATH})
-
-			### Calculate the python module path (using distutils)
-			execute_process ( COMMAND ${PYTHON_EXECUTABLE} -c "\
+	### FIND THE PYTHON INTERPRETER (AND THE SITE PACKAGES FOLDER)
+	if (UNIX AND NOT APPLE)
+		### Special-case for Debian's crazy, by checking to see if pybuild
+		### is available. We don't use it, except as a canary in a coal mine
+		find_program(PYBUILD_EXECUTABLE pybuild
+		             DOC "Path to Debian's pybuild utility")
+		if (PYBUILD_EXECUTABLE)
+			# We're on a Debian derivative, fall back to old path detection
+			set(py_detection "import site; print(site.getsitepackages()[0])")
+			set(PY_INSTALL_PREFIX "/usr/local") # An assumption (bad one?)
+		else()
+			# Use distutils to detect install path
+			set (py_detection "\
 from distutils.sysconfig import get_python_lib; \
-print( get_python_lib( plat_specific=True, prefix='${CMAKE_INSTALL_PREFIX}' ) )"
-				OUTPUT_VARIABLE _ABS_PYTHON_MODULE_PATH
-				OUTPUT_STRIP_TRAILING_WHITESPACE )
-
-			GET_FILENAME_COMPONENT(_ABS_PYTHON_MODULE_PATH
-					"${_ABS_PYTHON_MODULE_PATH}" ABSOLUTE)
-			FILE(RELATIVE_PATH _REL_PYTHON_MODULE_PATH
-					${CMAKE_INSTALL_PREFIX} ${_ABS_PYTHON_MODULE_PATH})
-			SET(PYTHON_MODULE_PATH ${_ABS_PYTHON_MODULE_PATH})
+print( get_python_lib( plat_specific=True, prefix='${CMAKE_INSTALL_PREFIX}' ) )")
+			set(PY_INSTALL_PREFIX ${CMAKE_INSTALL_PREFIX})
 		endif()
 	endif()
-	message("PYTHON_MODULE_PATH: ${PYTHON_MODULE_PATH}")
+
+	if (NOT PYTHON_MODULE_PATH)
+		execute_process ( COMMAND ${PYTHON_EXECUTABLE} -c "${py_detection}"
+		                  OUTPUT_VARIABLE _ABS_PYTHON_MODULE_PATH
+		                  OUTPUT_STRIP_TRAILING_WHITESPACE )
+
+		GET_FILENAME_COMPONENT(_ABS_PYTHON_MODULE_PATH
+		                       "${_ABS_PYTHON_MODULE_PATH}" ABSOLUTE)
+		FILE(RELATIVE_PATH _REL_PYTHON_MODULE_PATH
+		${PY_INSTALL_PREFIX} ${_ABS_PYTHON_MODULE_PATH})
+		SET(PYTHON_MODULE_PATH ${_REL_PYTHON_MODULE_PATH}
+		    CACHE PATH "Install path for Python modules (relative to prefix)")
+	endif()
+
+	message(STATUS "Will install Python module to: ${PYTHON_MODULE_PATH}")
 
 	############### INSTALL HEADERS & LIBRARY ################
 	### Install Python bindings