Use GNUInstallDirs to set installation directories

[Zopolis4]

Allow the user to override directories via typical options if necessary, otherwise use the GNU defaults.

Only change in behaviour is to install into ${prefix}/lib64 on systems where the following conditions are met.

  #  - we are on Linux system but NOT cross-compiling
  #  - we are NOT on debian
  #  - we are NOT building for conda
  #  - we are on a 64 bits system

Resolves #981

[Zopolis4]

It seems like the AppVeyor CI fail is unrelated to my changes.

[redboltz]

It seems like the AppVeyor CI fail is unrelated to my changes.

Yes, I think so. But our human resource is very limited. In this case, I ask you to fix it even if it is not related to your fix.
I've checked the appveyor error.
The error is caused by non exisitng link http://zlib.net/zlib-1.3.tar.gz
It should be http://zlib.net/zlib-1.3.1.tar.gz

Here is appveyor setup file:
https://github.com/msgpack/msgpack-c/blob/2c0040cef6e0cd9f871979bb6e6b57fa74de0d46/appveyor.yml

Please try replacing all 1.3 to 1.3.1.

[redboltz]

I just merged your appveyor update pull requests. Could you rebase this PR from the merged cpp_master ?

[codecov-commenter]

Codecov Report

Merging #1108 (d1af4a3) into c_master (5f287d1) will not change coverage.
The diff coverage is n/a.

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Additional details and impacted files

@@            Coverage Diff            @@
##           c_master    #1108   +/-   ##
=========================================
  Coverage     55.45%   55.45%           
=========================================
  Files             8        8           
  Lines          1044     1044           
=========================================
  Hits            579      579           
  Misses          465      465