Using a cross-compiler to build GNU Pascal results in a compiler binary that runs on the cross-target platform. This is called “crossbuilding”. A possible reason why anybody would want to do this, is when the platform on which you want to run the GNU Pascal compiler is not self-hosting. An example is mingw32.
To crossbuild GNU Pascal, you have to install a cross-compiler for your target first, see Cross-Compilers.
As when building a cross-compiler, configuration and compilation of the compiler can be done using the scripts cfgpc and mkgpc which are included in the source distribution in the subdirectory p/script. Please call them with the -h option for instructions.