TMVA fails to link without BLAS

[hahnjo]

Commit f3d5a1e turned off use_gsl_cblas by default, but tmva is on by default and requires BLAS. However, it seems CMake doesn't show an error and only linking will fail eventually, very late in the compilation process.

[guitargeek]

@ferdymercury, are you looking into this? Just to coordinate that we don't do duplicate work

[ferdymercury]

Nope, go ahead ;)
I'd say configure error message could be printed along these lines:
Either install openblas on system (preferred) or enable use_gsl_cblas and builtin_gsl (if Gsl not installed on system).