gnome.mkenums: Use rspfiles on Windows when possible

[amyspark]

This MR aims to update the work made in #4966 to allow (known) custom targets, like Gnome's glib-mkenums, to use response files.

I'm setting it as draft because this doesn't completely work on Windows; this is because I cannot find a suitable point in the target lifecycle to convert arguments to response files:

• Doing so in the Gnome module breaks placeholder variables conversion
• Doing so at the Ninja backend level breaks generation altogether (making the CUSTOM_COMMAND rule rspable means I have to supply a placeholder to rsp_content -- see below)
• Doing so at the generic backend level (either in as_meson_exe_cmdline or get_executable_serialisation) seems like the wisest choice, however on Windows scripts must go through an interpreter. In the case of glib-mkenums it's Python, but it's not possible to ensure I'm converting just the call to glib-mkenums, as the command + arguments have already been completely flattened

All feedback, and ideas to fix the last hurdle, is appreciated.

(EDIT -- I think there's still an outstanding bug, in that the space escaping doesn't work in some circumstances, yielding a broken interpreter call C:\Program instead of the full Python path. I need to sort out the response file handling before being able to determine where it happens and why.)

Fixes #6710

[amyspark]

cc @nirbheek