Configure runtime path linking when using shared libraries

[rafmudaf]

Feature or improvement description
This pull request configures the RPATH setting in CMake to configure where executables look for shared libraries when compiled with the BUILD_SHARED_LIBS setting enabled.

Related issue, if one exists
#430
#441

Impacted areas of the software
Any targets compiled with shared libraries

[andrew-platt]

Does this allow for dylibs to be ported between systems?

[rafmudaf]

The libraries would still have to be available in the destination-system's rpath, but otherwise yes. This removes the hard-coded path to dynamically linked libraries in executables.

[andrew-platt]

I'll check to see if this fixes the issues with the Simulink builds

[andrew-platt]

this doesn't seem to work out of the box for Mac OSX 10.15.7 with MATLAB 2020a

Caused by:
    Error using Run_OpenLoop (line 12)
    Invalid MEX-file '/Users/aplatt/software-development/GitHub/openfast-2/install/lib/FAST_SFunc.mexmaci64':
    dlopen(/Users/aplatt/software-development/GitHub/openfast-2/install/lib/FAST_SFunc.mexmaci64, 6): Library not loaded: @rpath/libopenfastlib.dylib
      Referenced from: /Users/aplatt/software-development/GitHub/openfast-2/install/lib/FAST_SFunc.mexmaci64
      Reason: image not found

However, if I setup the build directory with the cmake flags -DCMAKE_INSTALL_NAME_DIR=@loader_path -DCMAKE_MACOSX_RPATH=ON it works (sets @loader_path instead of @rpath).