Install the library into a multiarch path

[mapreri]

CMAKE_INSTALL_LIBDIR defaults to "lib" or "lib64" or
lib/, depending on the system.

CMAKE_INSTALL_INCLUDEDIR is pretty much always "include".

https://cmake.org/cmake/help/latest/module/GNUInstallDirs.html

Please take this patch so that distributors can more easily install the library into standard paths.

[jonjoliver]

Hi mapreri

The current install was in pull request #107 by cgull
I am happy to merge your pull request, but I want to make sure that it does not break cgull's (and related people) install
I am checking with him

Cheers
jono

[mapreri]

Sure! I'd also be happy to rework and adapt so it fits everybody's needs!

[jonjoliver]

The current official release is 4.8.2 which incorporates cgull's fix.
Lets give cgull an opportunity to reply.
And if he does not reply in a week, then we incorporate your pull request and do that as a pre-release
And see if anyone raises issues.

[cgull]

Sorry I missed this earlier. I'm working on FreeBSD and this change works fine there. @jonjoliver this is pretty clearly the right thing to do (another level of indirection!); go ahead and merge it. It might even help Cygwin or Haiku or something, too.

(On FreeBSD, lib is lib/ and include is include/, but both are in /usr/local for ports, but CMake already handles that in the install() function.)