Escape backslashes used in -D flags on Windows.

[TravisWhitaker]

Setup.hs as it is on master yields this cuda.buildinfo.generated on my Windows 10 machine (GHC 8.2.1, Stack 1.5.1):

buildable: True
cc-options: -DCUDA_INSTALL_PATH="C:\cuda\8\dev"
            -DCUDA_LIBRARY_PATH="C:\cuda\8\dev\lib/x64" -IC:\cuda\8\dev\include
ld-options: -LC:\cuda\8\dev\lib/x64
extra-libraries:
    cudart
    cuda
extra-ghci-libraries: cudart64_80
                      nvcuda
extra-lib-dirs: C:\cuda\8\dev\lib/x64
x-extra-c2hs-options: --cppopts=-E --cppopts=-m64 --cppopts=-DUSE_EMPTY_CASE

With this, stack build --stack-yaml=stack-8.2.yaml yields this:

c2hs.exe: C header contains errors:

.stack-work\\dist\\e53504d9\\build\\Foreign\\CUDA\\Path.chs.h:2: (column 24) [ERROR]  >>> Lexical error !
  The character '"' does not fit here.

This seems related to @yuhangwang's issue #48. Escaping the backslashes in the -D flags let's the build proceed, like so:

buildable: True
cc-options: -DCUDA_INSTALL_PATH="C:\\cuda\\8\\dev"
            -DCUDA_LIBRARY_PATH="C:\\cuda\\8\\dev\\lib/x64" -IC:\cuda\8\dev\include
ld-options: -LC:\cuda\8\dev\lib/x64
extra-libraries:
    cudart
    cuda
extra-ghci-libraries: cudart64_80
                      nvcuda
extra-lib-dirs: C:\cuda\8\dev\lib/x64
x-extra-c2hs-options: --cppopts=-E --cppopts=-m64 --cppopts=-DUSE_EMPTY_CASE

Strangely this only must be done with the -D flags; there's something strange among Windows, MinGW, and/or c2hs going on. This patch has Setup.hs escape the backslashes when on Windows. I believe it is always safe to do this, but there may be some Windows-specific edge case I'm missing. I'll also note that I get this error when building, although the build succeeds and the library seems to work:

In file included from .\cbits\stubs.h:9:0: error:
    0,
                     from cbits\init.c:1:
C:\cuda\8\dev\include/host_defines.h:84:0: warning: "__cdecl" redefined
 #define __cdecl

<built-in>: note: this is the location of the previous definition

In file included from .\cbits\stubs.h:9:0: error:
    0,
                     from cbits\stubs.c:5:
C:\cuda\8\dev\include/host_defines.h:84:0: warning: "__cdecl" redefined
 #define __cdecl

If all is well it'd be nice to get a point release with this fix, since I don't think there's a way to build this package on Windows otherwise (disregarding any Cygwin trickery).

[tmcdonell]

Awesome work, thanks!

Uploaded as cuda-0.8.0.1.