Promote GCC 4.7 to be the default compiler

This commit causes world and kernel to be built by GCC 4.7 if the compiler
is not specified.  As a side effect, the make variable "NO_GCC47" no longer
functions.  GCC 4.7 will always be built.  In addition to changing the
default of WORLD_CCVER, the CCVER variable also defaults to "gcc47" now.

The variable "NO_GCC44" has been resurrected to prevent GCC 4.4 from
being built.  As noted in the UPDATING file, using this option is highly
discouraged.  GCC 4.4 is very useful in building some packages in pkgsrc
that fail to build with GCC 4.7's stricter standards.  The new DPorts
system is designed to use GCC 4.4 by default, regardless of the base's
default compiler setting.  If at all possible, build *both* compilers
for the maximum flexibility to build packages.
Unbreak world with NO_GCC44

gcc44 in now optional, do not require its use during buildworld
anymore.