Be explicit about the library's name when building in Windows

We can't make everybody happy, so let's let them be happy themselves. If you need some weird prefix or filename for the libgit2 DLL, pass it as `-DLIBGIT2_FILENAME=` when building.

This supersedes #1639. I wanna make sure that this fits @csware / TortoiseGit and @nulltoken / libgit2#.