build: Make Python a required dependency.

Python 3 is required to generate the resources.qrc file.

Change-Id: Ifb82097183715e6a1a1008a1bb58272afd8019ec

CMakeLists.txt

@@ -30,6 +30,7 @@ file(GLOB_RECURSE
     ${PROJECT_SOURCE_DIR}/resources/*
 )
 include(FindPythonInterp)
+find_package(PythonInterp 3 REQUIRED)
 execute_process(
     COMMAND ${PYTHON_EXECUTABLE} ${PROJECT_SOURCE_DIR}/gen-resources.py
     WORKING_DIRECTORY ${PROJECT_SOURCE_DIR}