Install version/config files under CMAKE_INSTALL_LIBDIR

[jamessan]

Prior to the c_master/cpp_master split, this is where the files were installed (c.f., https://github.com/msgpack/msgpack-c/blob/6e7deb809120881634b3ca895e66b2a946084f34/CMakeLists.txt#L454) but this was changed to use "lib/" directly when updating the C++-only packaging.

Re-instate the use of CMAKE_INSTALL_LIBDIR so the user can control where they are installed and they follow the typical convention for location.

[codecov-commenter]

Codecov Report

Merging #1003 (b4d800e) into cpp_master (b350a00) will not change coverage. The diff coverage is n/a.

@@             Coverage Diff             @@
##           cpp_master    #1003   +/-   ##
===========================================
  Coverage       85.56%   85.56%           
===========================================
  Files              79       79           
  Lines            5009     5009           
===========================================
  Hits             4286     4286           
  Misses            723      723           

[redboltz]

Thanks you!