In C++ there are predefined macros:
#if defined(_M_X64) || defined(__amd64__)
// Building for 64bit target
const unsigned long MaxGulpSize = 1048576 * 128
You can also just define a symbol (e.g. _x64) in the project properties for the x64 platform. Open the properties dialogue of your project, select the x64 platform, on the Build page, just put "_x64" into the "Conditional compilation symbols" box.
Make sure to do this for both debug and release configuration.