CCVER: Add /usr/pkgsrc/lang/gcc-aux as 'gcc47'

CCVER: Add /usr/pkgsrc/lang/gcc-aux as 'gcc47'

There are two gcc-4.7.x packages in pkgsrc that will build gcc 4.7-based
compilers on Dragonfly.  One is located at lang/gcc47 and the other is
located at lang/gcc-aux.  Both have been patched by myself, but gcc-aux
seems to work better, especially with regards to link-time optimization.

Consequently, a new valid value for CCVER has been created, "gcc47", which
references lang/gcc-aux while building world and kernel.  Both will
compile, and the produced kernel will boot, but there are some network
issues.  Consequently, CCVER=gcc47 should only be used by developers who
have the express intent to fix any issues the arise from compiling
DragonFly sources with the latest GCC compiler.  Normal users looking
for non-base GCC compilers should stick with CCVER=gcc46 for now.