I need to multiply two polynomials each having small integral coefficients. I need a fast FFT routine in C/C++ which can convolve them. I have seen several libraries but they seem to be too large spread over multiple files. What is important is I need code which is not too long and can be very easily used and compiled in a single `.c/.cpp` file. 

1. FFT should be optimized for real inputs at least if not small integers.
2. Radix 4 implementation if available would be fine too.
3. Compiling it should take no special compilation flags as compilation of program has to be done in external environment which I can't control.

One that very well matches my needs is [here](http://www.koders.com/c/fid08FC2E44FE8165B7A590D69FF6B72D31634D426A.aspx). But I need something twice as fast.