ENH: Use GNUInstallDirs CMake module

[gdevenyi]

This is @cookpa's work. Given it fixes some outstanding bugs #872 we should merge.

Trying to enable installation to the correct lib dir for each platform.

This just needs the SuperBuild bits still adjusted, I will provide feedback.

[gdevenyi]

I believe this line: https://github.com/ANTsX/ANTs/blob/master/ANTS.cmake#L199

Needs to have "lib" changed to ${CMAKE_INSTALL_LIBDIR}

[cookpa]

I think I've set all the variables correctly, but CMAKE_INSTALL_LIBDIR does not get propagated to the ANTS-build. A workaround is to go to ANTS-build and then

cmake -D CMAKE_INSTALL_LIBDIR=lib64 .
make install

I'm trying to figure out how to make ANTS use the Superbuild value

[gdevenyi]

This is probably related to "make install" not working properly at the superbuild level, but rather needs to happen inside ANTS after the final build.

[cookpa]

I'm thinking it is independently detecting CMAKE_INSTALL_LIBDIR=lib for my machine. Trying to add it to superbuild vars

[cookpa]

OK I think that last commit does the trick, I can now build with VTK and send libs to lib64.

Update: I spoke too soon, I forgot to turn shared libs on. Trying again

[cookpa]

I think I may have finally cracked it. Things were going wrong because when CMAKE_INSTALL_LIBDIR was lib64, ITK libs were going to staging/lib but VTK libs were going to staging/lib64. Now both stage to lib but then install to ${CMAKE_INSTALL_LIBDIR}.

I would be grateful if anyone could test this on a non-Mac OS system.

[gdevenyi]

Builds and installs properly on Ubuntu 18.04