Add CXXFLAGS to allow compiling with 'constexpr'

Otherwise failed with following error on Xubuntu 12.10 x64:
../include/Mercator-0.3/Mercator/BasePoint.h:30:12: error: ‘constexpr’ does not name a type
note: C++11 ‘constexpr’ only available with -std=c++11 or -std=gnu++11