Reversed logic

Now needs PLUMED_INTERNAL_LAPACK/BLAS to use the internal version,
and by default uses external one. This makes it more coherent with
master branch.

Thus:

One can either use external libraries with:

CPPGLAFS=-D__PLUMED_INTERNAL_BLAS -D__PLUMED_INTERNAL_LAPACK

uses internal ones, whereas:

DYNAMIC_LIBS=-lwhatever_appropriate

uses external ones.