Using --with-libgit2=internal "make install" isn't correct

[Jeff Johnson]

Using ./devtool and --with-lbgit2=internal, autogen.sh invokes
    cmake .

This uses the implicit prefix=/usr/local instead of a better prefix.

The current best prefix will be installing the library into
    $prefix/lib/rpm/lib
(i.e. private to rpm itself, linked by -lrpmmisc)

But $prefix isn't known (or passed to) autogen.sh.

There's likely a better fix in RPM's top-level Makefile.am to run
    cmake .
at the end of ./configure where --prefix is well known.