cd fbc
make rtlib gfxlib2 TARGET=i686-w64-mingw32
sudo make install-rtlib install-gfxlib2 TARGET=i686-w64-mingw32
This should cross-compile the Win32 FB libraries using the
i686-w64-mingw32 toolchain and install them into the proper directory in
/usr/local. Again, here it is important to ensure that the used source code matches the version of the installed FB-linux setup.
To be completely safe and avoid FB version incompatibilities, you can build an entire FB setup from sources, including the Win32 cross-compiling libraries:
cd fbc
make
make rtlib gfxlib2 TARGET=i686-w64-mingw32
sudo make install
sudo make install-rtlib install-gfxlib2 TARGET=i686-w64-mingw32