It seems the lapack routine I took from gromacs are not threadsafe.
Indeed, routines slasq5 and dlasq5 contained static variables.
I removed all of them and everything seems to still work correctly.
This seems to solve the problem that @gtribello
found in his no-buffer branch.

Now __PLUMED_INTERNAL_LAPACK and __PLUMED_INTERNAL_BLAS are
used as synonimous macros, consistently with the fact that either
both lapack and blas are internal or external

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.

I fixed it so that it *Really* works with both internal and external
lapack/blas. The only drawback (I could not do it better) is that
both lapack and blas should be either internal or external.

One can either use external libraries with:
CPPGLAFS=-D__PLUMED_EXTERNAL_BLAS and -D__PLUMED_EXTERNAL_LAPACK
and
DYNAMIC_LIBS=-lwhatever_appropriate

or, with no flags, use internal libraries.

The two flags are independent (one can e.g. use external blas
with internal lapack)

(compiles faster)