Ensure the RPATH does not get blanked out during ESMX_App installation.

[theurich]

This PR ensures that the RPATH that is correctly encoded during the ESMX_App build process is not blanked out during the installation step. The CMake default is to remove the encoded RPATH in executables during installation. I assume this is to support LD_LIBRARY_PATH setting during run-time. However, for ESMF applications, and therefore ESMX, this is not a good idea. You typically encode the RPATH into the executable for good reason, so the correct shared library is found during loading without relying on LD_LIBRARY_PATH.

[danrosen25]

This is fine with me. How did you encounter the issue? I'm brainstorming on the ESMX application prototypes and testing.

[theurich]

I was working on a toy code that itself has a dependency on PnetCDF. I satisfied this dependency in the toy model's CMakeLists.txt file (since my local ESMF build was not done with PnetCDF). The PnetCDF on my system is a shared library, and everything all the way up to linking the build/esmx_app was fine, setting the appropriate RPATH in the executable. However, during the install step into install/bin/esmx_app, the RPATH was blanked out.
The result was that build/esmx_app could execute fine, while install/bin/esmx_app was not finding the PnetCDF library during loading.
The proposed change keeps the RPATH intact during for install/bin/esmx_app.