CMakeLists.txt: pass headers as arguments to gen-def.py; call using PYTHON_EXECUTABLE

CMakeLists.txt

@@ -836,7 +836,7 @@ if (UNIX OR MINGW)
     # generate harfbuzz.def after build completion
     string(REPLACE ";" " " space_separated_headers "${project_headers}")
     add_custom_command(TARGET harfbuzz POST_BUILD
-      COMMAND ${CMAKE_COMMAND} -E env "headers=${space_separated_headers}" python ${PROJECT_SOURCE_DIR}/src/gen-def.py ${PROJECT_BINARY_DIR}/harfbuzz.def
+      COMMAND "${PYTHON_EXECUTABLE}" ${PROJECT_SOURCE_DIR}/src/gen-def.py ${PROJECT_BINARY_DIR}/harfbuzz.def ${space_separated_headers}
       WORKING_DIRECTORY ${PROJECT_SOURCE_DIR}/src)
 
     add_test(NAME check-static-inits.sh