Make library work on OSX

[rustomax]

Great work on nim-libgd! I've tested this library on OSX and it works. The only adjustment that needs to be made is the procedure definition pragmas in libgd.nim should reference libgd.dylib library name instead of libgd.so, for instance

proc gdImageCreate(sx: cint; sy: cint): gdImagePtr {.cdecl, importc: "gdImageCreate", dynlib: "libgd.dylib".}

Could you write a compile-time check for this that injects appropriate library name?

Additionally brew install libgd will get the C library installed on OSX.

[rustomax]

See PR [#2]

[mrhdias]

Thank you for your help. I already committed the pull request.