GF_LIB_PATH can now be path1:path2:path3, not just path1

Traditionally, GF_LIB_PATH points to something like
`.../share/ghc-8.0.2-x86_64/gf-3.9/lib`

and if you want prelude and alltenses and present, you add a
`--# -path=.:present`
compiler pragma to the top of your .gf file

But if you are developing some kind of application grammar
library or contrib of your own, you might find yourself
repeating your library path at the top of all your .gf files.

After painstakingly maintaining the same library path at the
top of all your .gf files, you might say, let's factor this
out into GF_LIB_PATH.

Then you might then find to your surprise that GF_LIB_PATH
doesn't accept the usual colon:separated:path notation
familiar from, say, unix PATH and MANPATH.

This patch allows you to define
`GF_LIB_PATH=gf-3.9.lib:$HOME/gf-contrib/whatever/lib`
in a more natural way.

If you are an RGL hacker and have your own version of the
RGL tree sitting somewhere, you should be able to have both
paths in the GF_LIB_PATH, for added convenience. This minor
convenience will probably lead to obscure bugs and great
frustration when you find that your changes are mysteriously
not being picked up by GF; so keep this in mind and use it
cautiously.

This caution should probably sit in the documentation
somewhere. A subsequent commit will do that.

If you use zsh, you can do this to quickly build up a big
GF_LIB_PATH:

% gf_lib_path=( $HOME/src/GF/lib/src/{api,abstract,common,english,api/libraryBrowser,prelude,..} )

% typeset -xT GF_LIB_PATH gf_lib_path

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