Install manifold.pc under CMAKE_INSTALL_LIBDIR

[cjmayo]

On platforms that install libmanifold under lib64, manifold.pc should be installed in the pkgconfig directory under lib64.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.25%. Comparing base (d437097) to head (5bc74d5).
Report is 100 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #946      +/-   ##
==========================================
- Coverage   91.84%   88.25%   -3.59%     
==========================================
  Files          37       62      +25     
  Lines        4976     8668    +3692     
  Branches        0     1049    +1049     
==========================================
+ Hits         4570     7650    +3080     
- Misses        406     1018     +612     

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[pca006132]

Btw did you find any other hardcoded paths in our other cmake files? Ideally those should be changed as well, if any.