Win32: Allow to use git.rc without cmake again

https://github.com/libgit2/libgit2/commit/095bfd748766966f5515bdfe64867d6a09287123 complicates building of libgit2 if no CMake is used (e.g. in TortoiseGit where the libgit2 project is used as a submodule and a static VisualStudio project file is used). So everyone who wants to build TortoiseGit has to run CMake (and gains nothing, since the generated project files are of no use).