Please add to CMakeLists.txt option for CHIPMUNK_FFI

[nemysis]

I try to adjust Chipmunk-6.2.0 that works proper in FreeBSD.

http://www.freebsd.org/cgi/query-pr.cgi?pr=ports/180061

And to port pymunk which use Chipmunk

http://www.freebsd.org/cgi/query-pr.cgi?pr=ports/170662

I have tried to use -DCHIPMUNK_FFI but see this is only for XCode or OS X.

xcode/Chipmunk6.xcodeproj/project.pbxproj
...
                                OTHER_CFLAGS = (
                                        "-DDEBUG",
                                        "-DCHIPMUNK_FFI",
                                        "-ffast-math",
                                );

The compile should define CHIPMUNK_FFI as

//Build the Chipmunk FFI
BUILD_CHIPMUNK_FFI:BOOL=ON

Now i have in Makefile for Chipmuk

CMAKE_ARGS+=    -DBUILD_CHIPMUNK_FFI:BOOL=ON

and get

CMake Warning:
  Manually-specified variables were not used by the project:

    BUILD_CHIPMUNK_FFI

I have made a pull request that enables this feature. Demos still need to be updated, though.