Ignoring "empty macro arguments" warnings from the clang compiler.

Note: this warning only pops up if your compiler doesn't support C++11
or if you configure with --disable-cxx11.

Refs #329
Rebuilt the fpoptimizer.cc file using GCC-4.6 on Linux.

This generated a 9k line diff, so I'm not sure exactly what changed,
but the new pragmas for suppressing warnings definitely made it in.

Refs #329.
Merge pull request #332 from libMesh/fparser_ignore

Ignore clang compiler warnings in fparser