Set RPATH correctly for the targets

[thilinarmtb]

Description

In the current CMake build, OCCA doesn't add install path to RPATH. This PR adds
the install path to RPATH and one can do the following without having to set the
LD_LIBRARY_PATH.

${CMAKE_INSTALL_PREFIX}/bin/occa info

In the current development version, one has to do:

export LD_LIBRARY_PATH=${CMAKE_INSTALL_PREFIX}/lib:${LD_LIBRARY_PATH}
${CMAKE_INSTALL_PREFIX}/bin/occa info