[cmake] Slight fix ups to make robust to the full range of GNUInstallDirs

See https://cmake.org/cmake/help/v3.14/module/GNUInstallDirs.html#result-variables for `CMAKE_INSTALL_FULL_*`

Reviewed By: sebastian-ne

Differential Revision: https://reviews.llvm.org/D130545

GitOrigin-RevId: 28e665fa054d62d4e2c777774cc83dea533dfe6e
2 files changed