Make pygit work (for FAForever) in a frozen environment

Two things here:
  - Ensure git stores headers with LF line endings, so it's parseable by CFFI at runtime even on windows.
  - If sys has attribute 'Frozen', we need to look in a different place for the decl.h file

I'm not sure you'll want this in main, but it posed problems for me when using pygit2 in a frozen executable (Using cx_Freeze), where the python modules are loaded from a .zip file.