Need support for dealing with additional options needed by RVCT4 that are not valid for RVCT2.2

RVCT4 requires additional options to the compiler to get the behaviour we require to link Qt and Symbian DLLs. Right now, this is --import_all_vtbl, but other keywords are coming to solve eg. the webkit udeb linking problem.

We need a mechanism to add these keywords to the compiler command line if we are building with RVCT4 (or newer), but not with RVCT2.2, where they would obviously cause build errors as they're not valid options to the 2.2 compiler.