scripts: add a way to provide libclang include dirs externally

[rmelotte]

When cross-compiling, libclang doesn't necessarily have the right include directories configured by default.
See for example: https://bugs.ghostscript.com/show_bug.cgi?id=708041

Since we also have no way to figure out the right include directories from our scripts, add a new environment variable that can be used to provide them externally.

[julian-smith-artifex-com]

Many thanks for this PR.

Some minor suggestions:

1. Could you call the environment variable MUPDF_LIBCLANG_ARGS? as it isn't inherently limited to include directories.
2. Use shlex.split() instead of splitting simply by spaces, in case we ever need to pass args with spaces etc.
3. We tend to use os.environ.get() instead of os.getenv().

So something like:

import shlex
...
libclang_args = os.environ.get('MUPDF_LIBCLANG_ARGS', '')
args += shlex.split(libclang_args)

Some non-technical things:

1. To accept your PR we'll need you to agree to our Artifex Contributor License Agreement - see:
   https://artifex.com/contributor/

2. This Github repostitory is really a readonly copy of the main repository ghostscript.com:/home/git/mupdf.git, so when your PR is ok i will push your commit to the main repository manually instead of merging the PR here.

3. If you prefer, i can create a commit independently to save you from worrying about the CLA etc.

[rmelotte]

Thanks for your excellent suggestions, I applied them and updated the PR.

Regarding the CLA, I actually already signed it using the CLA Assistant in the PyMuPDF project in pymupdf/PyMuPDF#3173.
Do I need to sign it again?

[rmelotte]

Hi @julian-smith-artifex-com ,
Did you get information regarding the CLA? Do I need to sign it again?

[julian-smith-artifex-com]

Apologies for the delay. Yes, you've already signed the CLA so there's no need to do so again.

I'm off work for the next few days, but i've added a todo item to submit your commit for internal review, hopefully by the end of next week.