Change from -W to -Wextra for the GenericGCC module

Since GCC 3.4 (of April 2004 [1]), the more descriptive -Wextra has been accepted in lieu of -W [2]. I suggest that versions of GCC <= 3.3 (from 2003 and before) are not useful in the context of QBS, and that the GenericGCC module should therefore use the -Wextra flag (also accepted by clang [3]) for clarity.

[1] http://gcc.gnu.org/releases.html
[2] http://gcc.gnu.org/onlinedocs/gcc-3.4.6/gcc/Warning-Options.html#Warning-Options
[3] http://programmers.stackexchange.com/a/124574