Well, I did some tinkering around. Using the Make provided by FPC builds the compiler. Problem is that it seems to install every utility the installer build came with, making me assume it will rely on those, as well as putting the in a <cpu>-<target> subdirectory of bin/ with everything else in toplevel rather than lib/ or share/doc
I'm looking to simply bootstrap the compiler with MinGW-w64 binutils backend with just the RTL, or minimal needed RTL. After that, I want to build the actual 32 and 64 bit compiler packages in full.