build: Actually search for Python 3 interpreter before using it.

CMakeLists.txt

@@ -43,6 +43,17 @@ find_package(cJSON)
 find_package(Systemd)
 find_package(OpenGLES COMPONENTS V3)
 
+if(NOT CMAKE_VERSION VERSION_LESS 3.12)
+	find_package(Python3 REQUIRED Interpreter)
+	set(PYTHON_EXECUTABLE Python3::Interpreter)
+else()
+	find_package(PythonInterp REQUIRED VERSION 3)
+	if(PYTHON_EXECUTABLE MATCHES "WindowsApps")
+		# If you hit this error, you will have to install Python 3 or try harder to tell CMake where it is.
+		message(FATAL_ERROR "Found WindowsApps alias for Python. Make sure Python3 is installed, then choose 'Manage App Execution Aliases' in Start and disable the aliases for Python.")
+	endif()
+endif()
+
 add_library(xrt-pthreads INTERFACE)
 if(WIN32)
 	find_package(pthreads_windows REQUIRED)