BLD: Don't specify runtime_library_dirs on Cygwin

[DWesl]

runtime_library_dirs isn't supported by PE/COFF, used by Windows and Cygwin; it seems to be an ELF thing.

• Closes Unable to install using pip on Cygwin #1113, Cygwin currently does not support --enable-new-dtags pypa/distutils#171
• Tests added
• Fully documented, including history.rst for all changes and api/*.rst for new API

[codecov]

Codecov Report

Merging #1120 (0083868) into main (6147e5d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1120   +/-   ##
=======================================
  Coverage   96.15%   96.15%           
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  Files          20       20           
  Lines        1794     1794           
=======================================
  Hits         1725     1725           
  Misses         69       69           

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[snowman2]

Thanks @DWesl 👍