GCC Canadian Cross
A Canadian Cross is to first build a cross-compiler for system Y (the host) on system X (the build-system). You would then use that cross-compiler, setting
--build=<where this compiler runs> --host=<where the compiler you're building shall run> --target=<where the executables the compiler you're building will build shall run>. This explanation alone should be sufficient to show why this is both non-trivial and error-prone.
"Minor" Canadian Cross
Building a cross-compiler takes some time, so you might want to use a faster machine (like, a PIV) to build a cross-compiler for a lesser machine (like, a Pentium laptop). You don't really need the initial cross-compiler for this, as the system compiler on the build machine (PIV) can generate binaries for the host machine (Pentium) alright.