Complex sentence: Blocks uses a custom build system, which stores its information in XML-based project files, but it can optionally use external makefiles, which simplifies interfacing with projects using the GNU and Qt Software's qmake build systems.
Split into simpler sentences: Blocks uses a custom build system, which stores its information in XML-based project files. It can optionally use external makefiles, which simplifies interfacing with projects using the GNU and Qt Software's qmake build systems.