Install mopac-bz only if BUILD_WINMOPAC is set

[awvwgk]

Building on Windows without BUILD_WINMOPAC enabled results in a missing install target mopac-bz.

CMake Error at CMakeLists.txt:283 (install):
  install TARGETS given target "mopac-bz" which does not exist.


-- Configuring incomplete, errors occurred!

Status

• Ready for merge

[codecov-commenter]

Codecov Report

Merging #63 (d9782b3) into main (5edb088) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #63   +/-   ##
=======================================
  Coverage   68.24%   68.24%           
=======================================
  Files         330      330           
  Lines       71703    71703           
=======================================
  Hits        48932    48932           
  Misses      22771    22771           

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[godotalgorithm]

Thank you, this change is necessary to compile on Windows without the Intel Fortran compiler and its QuickWin library.