Check for existence of libraries before parsing their flags with pkg-config

[madmiraal]

As requested here:

godot/platform/linuxbsd/detect.py

Line 232 in 8f6cba3

# FIXME: Check for existence of the libs before parsing their flags with pkg-config

This PR basically replaces:

if not env["builtin_<library>"]:
    `env.ParseConfig("pkg-config <library> --cflags --libs")`

with

if not env["builtin_<library>"]:
    error = os.system("pkg-config <library> --exists")
    if error:
        print("Error: <library>-dev library not found. Using builtin <library>.")
        env["builtin_<library>"] = True
    else:
        env.ParseConfig("pkg-config <library> --cflags --libs")

When there are dependencies it checks for all the libraries and only if they are all requested and all present does it use the shared libraries. It also provides useful information about which libraries are missing and which were not requested.

When there is a minimum version required the check is changed to:

error = os.system("pkg-config <library> --atleast-version='<version>'")

godot/platform/linuxbsd/detect.py

Lines 281 to 282 in 8f6cba3

	if False: # not env['builtin_assimp']:
	# FIXME: Add min version check

I've removed the check for assimp instead of fixing it, because it was removed in #44599 (#42941 in 3.x).

I've made all the checks consistent (including the initial ones in can_build()).

The main difference this PR makes is that when pkg-config <library> --exists returns an error, it reports the error and then falls back to using the builtin library instead of failing and not building at all. See #56526 and #7373.

[Riteo]

Oof this one limbo'ed hard.

Yeah the TODO's still there and the logic looks very similar. It will still probably require a good rebase though.

Looks a bit verbose to me but considering the rest of the script and the specific requirements that some dependencies have I can't really see a worthwhile solution.