Add compiler:c++11-lang to USES to fix build on GCC-based systems:
CMake Error in CMakeLists.txt: The compiler feature "cxx_decltype" is not known to CXX compiler "GNU" version 4.2.1.
While here, sort USES; add one blank line to be in accordance with
common usage.
Approved by: portmgr (tier-2 blanket)