Support for cross-compiling using -d:mingw
Currently, when attempting to cross-compile anything that uses a nimterop wrapper from Linux to Windows, a compilation error like following occurs:
```
stack trace: (most recent call last)
../src/rapid/lib/freetype.nim(33, 10) freetype
../../../../.nimble/pkgs/nimterop-0.1.0/nimterop/git.nim(101, 8) gitPull
../../../../.nimble/pkgs/nimterop-0.1.0/nimterop/git.nim(27, 23) mkDir
../../../../.nimble/pkgs/nimterop-0.1.0/nimterop/git.nim(21, 11) execAction
../../../../.choosenim/toolchains/nim-#devel/lib/system/assertions.nim(27, 20) failedAssertImpl
../../../../.choosenim/toolchains/nim-#devel/lib/system/assertions.nim(20, 11) raiseAssert
../../../../.choosenim/toolchains/nim-#devel/lib/system/fatal.nim(39, 5) sysFatal
/home/daknus/.choosenim/toolchains/nim-#devel/lib/system/fatal.nim(39, 5) Error: unhandled exception: /home/daknus/.nimble/pkgs/nimterop-0.1.0/nimterop/git.nim(21, 12) `ret == 0` Command failed: (127, false)
ccmd: cmd /c mkdir  \home\daknus\Coding\Nim\rapid\src\rapid\lib\freetype_src
result:
/bin/sh: cmd: command not found
```