meson: Don't call find_program objects with python

[nirbheek]

On UNIX-like OSes, the OS will read the shebang and use the correct interpreter, and on Windows, Meson will read the shebang and use the correct interpreter.

Adding it manually will cause python to try to interpret python.exe

With this change, libepoxy builds and the tests pass with MSVC on Windows for me.

[ebassi]

Interesting; I did add the explicit Python call because of MSYS2 build errors. Can you confirm that it still builds with Meson 0.37 on MSYS2?

[nirbheek]

I tested this on native Windows and on MSYS, but not on MSYS2 since I don't use it. However this is how we do things in GStreamer too and other people have tested that it works there. Will confirm and report back though!

You might also see a PR adding AppVeyor CI support to build with MSVC and MinGW. :)

[nirbheek]

So it doesn't work on MSYS2, because there Python 3.5 is called python3.exe and python3.5.exe but not python.exe. So #!/usr/bin/env python in the gen_dispatch.py can't find python.

The fix is to use #!/usr/bin/env python3 as the shebang, but that will not work on native Windows with Meson 0.37.1 since Python 3 installs itself as python.exe with the Windows installer. However, Meson 0.38 will correctly use sys.executable on Windows when it encounters python3 in a shebang, so it should be ok to do that in the long run.

[nirbheek]

I will note that this same problem also occurs on Linux distros since no one ships Python 3 as python.exe. Essentially Python 2 is required on all platforms for building libepoxy, and if you install Python 2 inside MSYS2 it builds just fine.

[ebassi]

Ideally, no: Epoxy should build with Python 3.

The only reason why Python 2 may be used is because of older, enterprise-y environments — see issue #43.

I guess we need to change gen_dispatch.py to take the right version of Python in the shebang line, i.e. using something like:

#!/usr/bin/env @PYTHON@

and then replacing @PYTHON@ with either python3 or python depending on the environment. Older OSes can still use the autotools build until they ship with Python 3.4.

[nirbheek]

I was hoping there's a way to avoid that, but such is life. ;)

[ebassi]

Okay, so I kind of changed my mind, given that Epoxy has to build on platforms without Python 3, which means we're going to keep gen_dispatch.py compatible with Python 2 for the foreseeable future.

I'll merge this PR, and then update the documentation to say that we require Python 2 for building Epoxy.