Obey CMAKE_INSTALL_PREFIX

[bryanloz-xilinx]

Problem solved by the commit

If you want to install XRT into a user specified directory, XRT's CMake infra will not obey the user.
It will always put it into CMAKE_INSTALL_PREFIX/xrt

This change would make the CMake infra more obedient to the user.

Risks (if any) associated the changes in the commit

Could potentially break existing flows.

[bryanloz-xilinx]

I'm only 90% sure I want to make this happen. This PR also serves as a test to see if this change breaks anything.

[gbuildx]

Build failed :(

[gbuildx]

Build failed :(

[stsoe]

This indeed breaks many different repos that use XRT as a submodule.

Also, CMake itself prepends CMAKE_INSTALL_PREFIX to all variables referenced in install directories so now you have two levels of CMAKE_INSTALL_PREFIX.

I agree it should be possible to set CMAKE_INSTALL_PREFIX to something different from what is hardwired, but the change in this PR will not do it.

Can we close and schedule a more thorough revamp of XRT's CMake infra.? It's long been on my wish list, but no resources assigned.

[stsoe]

I'm only 90% sure I want to make this happen. This PR also serves as a test to see if this change breaks anything.

Please do all testing outside the build pipeline in your own environment.

[bryanloz-xilinx]

@stsoe Closing in favor of CMake revamp