build: fix invoking scan_enums.py

The "scan_enums.py" script requires a Python v2 interpreter to be
available. The "CMakeLists.txt" simply searched for a Python
interpreter, which failed on modern Debian systems.

Instead of simply searching for "python", the CMake script was adjusted
to specifically search for a Python 2 interpreter, and then to
explicitly invoke that to run the "scan_enums.py" scripts.

Furthermore, a dependency was created to ensure that the script runs
before the "core-libs" package is compiled, and thus to ensure that any
compilations use the newly-generated versions of the files.

An improvement would be to remove "enum_tables.h" and "enum_tables.cpp"
from the repository, generate the files into the "build" directory, and
compile and link against those, although that would mean every developer
would need to have Python installed. This is currently optional as long
as no new enums are added which need to be accessed by Lua.

CMakeLists.txt

@@ -357,11 +357,15 @@ if (NATURALDOCS)
 	WORKING_DIRECTORY ${PROJECT_SOURCE_DIR})
 endif()
 
-find_program(PYTHON NAMES python)
-if (PYTHON)
+find_package(Python2 COMPONENTS Interpreter)
+if (Python2_Interpreter_FOUND)
 	add_custom_target(enums
-		COMMAND scripts/scan_enums.py -o src/enum_table.cpp --pattern='*.h' -r src
-		WORKING_DIRECTORY ${PROJECT_SOURCE_DIR})
+		COMMAND "${Python2_EXECUTABLE}" scripts/scan_enums.py -o src/enum_table.cpp --pattern='*.h' -r src
+		WORKING_DIRECTORY ${PROJECT_SOURCE_DIR}
+	)
+	add_dependencies(pioneer-core enums)
+else()
+	message(WARNING, "Python 2 not found; enums will not be scanned.")
 endif()
 
 target_link_libraries(pioneer-lib PUBLIC lz4 fmt::fmt)