Added mergesort benchmark

    * Makefile.am: Added mergesort benchmark
    * Makefile.in: Regenerated
    * configure: Regenerated
    * configure.ac: Added mergesort benchmark
    * src/mergesort/Makefile.am: Added mergesort benchmark
    * src/mergesort/Makefile.in: Regenerated
    * src/mergesort/mergesort.c: Added mergesort benchmark