Add numpy dependency fallback necessary for cross-compilation builds

[jcapriot]

Most cross-compilation builds are not able to make use of meson's numpy_config script to find the necessary header files, (See conda-forge/geoana-feedstock#34 where I was testing this before releasing a new version.)

I wasn't quite sure if it was able to do it or not (but I suspected it wasn't). So this adds a fallback method of determining the numpy include directory incase the normal lookup fails. So this addition follows what pywt does. (see here: https://github.com/PyWavelets/pywt/blob/361cc9a4c80bc2603fae99e151f2a391c0b376f9/pywt/_extensions/meson.build#L13)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.88%. Comparing base (979cf23) to head (e9a7d8d).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #77   +/-   ##
=======================================
  Coverage   85.88%   85.88%           
=======================================
  Files          31       31           
  Lines        3139     3139           
=======================================
  Hits         2696     2696           
  Misses        443      443           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.