The newest version of this macro changes the way you include it, which is
a mess because we don't know which version a particular distro will have.
We copy the old version of the macro into the m4/ directory and include
it in git. That should prevent aclocal from installing the newer version.

(It would be better to include the newer version, but that has a bug that
clobbers the GITIGNORE variable.)

Closes #149