scons: Bump the compiler version to gcc 4.6 and clang 3.0

scons: Bump the compiler version to gcc 4.6 and clang 3.0

This patch bumps the supported version of gcc from 4.4 to 4.6, and
clang from 2.9 to 3.0. This enables, amongst other things, range-based
for loops, lambda expressions, etc. The STL implementation shipping
with 4.6 also has a full functional implementation of unique_ptr and
shared_ptr.