Allow compilation of Gaudi with clang 3.7 using gcc 4.9 externals

Compilation of Gaudi with clang 3.7 using the gcc4.9 externals (see LBCORE-934).

To do so some arguments have to be passed to cmake, assuming CMTCONFIG is x86_64-slc6-clang3.7-opt

• LCG_SYSTEM: To specify the flavour of the externals to use
• LCG_FORCE_COMPILER: To specify the compiler to use, with the format "-" (an lcg wrapper has to exist for this to work).

Using the LHCb Makefiles, you just need to specify:

export CMAKEFLAGS="-DLCG_SYSTEM=x86_64-slc6-gcc49 -DCMAKE_EXPORT_COMPILE_COMMANDS=1 -DLCG_FORCE_COMPILER=clang-3.7"