Fix CMP0053 warning with CMake 3.1.
Review Request #121971 - Created Jan. 10, 2015 and submitted
With CMake 3.1, @-style variable expansion is deprecated outside of
configure_file() or string(CONFIGURE) contexts (CMP0053). This causes
ecm_generate_pri_file() to produce dev warnings with this verison of
CMake, and would break this function in projects that set CMake 3.1 as
their required version.
Attica still configures and builds, and produces the same .pri file it did before.