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.
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
```
Run `make -f Makefile.embed` to get a `libgit2.a` with no dependencies
and no configuration on any Unix-like system.
The generated library can be linked with any piece of software without
licensing issues.
Have fun.