fontforge_package_name is used for defining the datadir path.

If you do not define it in the configure.ac then you will end up with
/usr/local/share/fontforge_package_name
instead of
/usr/local/share/fontforge

As there are locale, doc etc that are at the parent level to share/fontforge
I'm not 100% sure that having a version number in there would help.

If you have two versions of fontforge you wanted to parallel install then you
would still get conflicting files for
/usr/local/share/doc/fontforge/index.html

If on the other hand you use
./configure --prefix=/usr/local/fontforge-2014jan
then all would be installed into that subtree and you can have as many fontforges
as you like installed without using /usr/local/share/fontforge_package_name.

I'm not entirely sure about the change I'm reverting here. I imagine
that having a proper version in there instead of
fontforge_package_name might be what was intended?
Merge pull request #1078 from monkeyiq/2014jan/ff-share-fontforge-dirname

fontforge_package_name is used for defining the datadir path.