move handling of F77 man page to docs directory, add doxygen build

[edhartnett]

The man page source (netcdf.m4) is in docs, but it is being turned into the F77 man page in libsrc, which is where the old cfortran.h based F77 API was, but is no longer.

So I will move handling of the F77 man page into the doc directory.

Also, even with --enable-doxygen the documentation does not build with make all. Doxygen is found but not used. I have put the stuff needed in docs/Makefile.am to build the documentation when --enable-doxygen is used with configure.