Ensure that Python3 is available

Use the more standard cmake find_package to search for Python3 and make it required. This also provides
a better error message than "version.cmake" not found when python3 is missing.

Change-Id: I350fd615ed8474d34392a057a5f8bded78173949

CMakeLists.txt

@@ -107,9 +107,9 @@ else ()
     endif ()
 endif ()
 
-find_program(PYTHON NAMES python3 python)
+find_package(Python3 REQUIRED COMPONENTS Interpreter)
 execute_process(
-    COMMAND ${PYTHON} ${CMAKE_CURRENT_SOURCE_DIR}/contrib/cmake/parse-version.m4.py ${CMAKE_CURRENT_SOURCE_DIR}/version.m4
+    COMMAND ${Python3_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/contrib/cmake/parse-version.m4.py ${CMAKE_CURRENT_SOURCE_DIR}/version.m4
     WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
     )
 include(${CMAKE_CURRENT_BINARY_DIR}/version.cmake)
@@ -339,7 +339,7 @@ include_directories(${CMAKE_CURRENT_SOURCE_DIR} src/compat include)
 
 add_custom_command(
     OUTPUT always_rebuild config-version.h
-    COMMAND ${PYTHON} ${CMAKE_CURRENT_SOURCE_DIR}/contrib/cmake/git-version.py
+    COMMAND ${Python3_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/contrib/cmake/git-version.py
     )
 set(HAVE_CONFIG_VERSION_H YES)