Complex sentence: XCOM compiles from XPL source code, but since XCOM itself is written in XPL it can compile itself--it is a "self-compiling compiler", not reliant on anyone else's compilers.
Split into simpler sentences: XCOM compiles from XPL source code, but XCOM itself is written in XPL too. It is thus a self-compiling compiler, not reliant on anyone else's compilers.