ENH: simplify cpp rules to avoid absolute paths and multiple GFLAGS.

- otherwise /lib/cpp may need a different library binding than
  currently available and results in this type of error:

  /usr/lib64/gcc/x86_64-suse-linux/4.4/cc1:
  /data/app/OpenFOAM/ThirdParty-1.6.x/platforms/linux64/gcc-4.3.3/lib64/libstdc++.so.6:
  version `GLIBCXX_3.4.11' not found (required by
  /usr/lib64/libppl_c.so.2)

ENH: remove unused flex++ rule for SiCortex that was identical to the
general one anyhow.