meson: use install_subdir

[zoziha]

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Description

• When minpack is the main meson project, using install-mod.py to install the *.mod file, meson install will be expected.
• When minpack is a meson subprojects project, installing the *.mod file using install-mod.py will have an unexpected effect.

The solution is to use meson's install_subdir, which I have used in several of my own projects and it meets all the requirements for a normal *.mod install.

[codecov[bot]]

Codecov Report

Merging #79 (0627e39) into main (c0b5aea) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #79   +/-   ##
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  Coverage   88.80%   88.80%           
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  Files           2        2           
  Lines        1224     1224           
  Branches      456      456           
=======================================
  Hits         1087     1087           
  Misses         40       40           
  Partials       97       97