Introduce variables PDDIR, PDINCLUDEDIR, PDBINDIR, PDLIBDIR
Introduce variables PDDIR, PDINCLUDEDIR, PDBINDIR, PDLIBDIR

These new path variables are aliases of existing ones that didn't have
coherent names. The new variables are all defined such that they are
overridable from the environment, as indicated by the upper case syntax.

ArnaudCourcelle/external,myQwil/pd-externals