upgrade the implementation of arch.path files

[Ehouarn Millour]

To better match with other models (ORCHIDEE, NEMO, ...), it would make sense to have the arch.path follow the same rules, i.e. definitions using 3 sets of keywords :

SOMETHING_LIBDIR ="-L....."
SOMETHING_LIB="-l..."
SOMETHING_INCDIR="-I...."

Of course thins implies adapting the makelmdz[_fcm] scripts accordingly.

An important benefit of having matching arch.path is that the file can then be shared by all models when compiling a combination (e.g. LMDZ+DYNAMICO+XIOS+IOIPSL)