Fix link pkg-config link dependency order

move Fortran library dependencies from cpgplot.pc to pgplot.pc

The Fortran library dependencies were erroneously put in the Libs
entry in cpgplot.pc rather than pgplot.pc.  This didn't affect
the internal tests.

However, the result was that -lgfortran was placed before -lpgplot
(for example), and on systems which perform single pass symbol lookup
during linking (e.g. most recent Linux boxes), the symbols in gfortran
which were required by pgplot were never resolved.