Allow compilation of Gaudi with clang 3.7 using gcc 4.9 externals

To do so, some arguments have to be passed to cmake, assuming
CMTCONFIG is x86_64-slc6-clang37-opt
  * LCG_SYSTEM: To specify the flavour of the externals to use
  * LCG_FORCE_COMPILER: To specify the compiler to use, with the
    format "<compiler>-<version>" (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"
```

Fixes GAUDI-1123 and LBCORE-934.

See merge request !48