Allow Makefile.embed to be used when cross-compiling

This allows libgit2 to be cross-compiled (e.g. when building native rugged binaries for windows from Linux or OS X).

```
CROSS_COMPILE=i686-w64-mingw32 make -f Makefile.embed
```
When building with MINGW, specify `__USE_MINGW_ANSI_STDIO`.

This option is already present in the CMake config, but was missing from `Makefile.embed` and would cause all kinds of weird failures when compiling rugged on windows with the ruby devkit.
Merge pull request #1748 from arthurschreiber/patch-1

Allow Makefile.embed to be used when cross-compiling