[RegularExpressions] Add support to build build with Mono.

This patch does the following things:

* Adds support to build without Code Access Security, by using the new
  define DISABLE_CAS_USE.  The reason is that this is both slow, and
  Mono does not actually offer a Code Access Secured environment.  So
  avoid paying the price.

* Removes some warnings from the build for unused variables.

* Allows the code to be bootstrapped with a 2.0 compiler (which Mono
  does during the bootstrap phase currently).