Delete bundled gcc libs on windows

This prevents having mismatches between bundled libs and libs on the system
(Cargo requires MinGW to compile regardless).
Bump libgit2 dep to hopefully fix win32
auto merge of #1010 : alexcrichton/cargo/mingw, r=brson

This prevents having mismatches between bundled libs and libs on the system
(Cargo requires MinGW to compile regardless).