Update the Clang pattern to specify compiler options.

In Clang 3.4, the preferred (and new) option pattern is --target=.

 http://thread.gmane.org/gmane.comp.compilers.clang.devel/29975
 http://llvm.org/viewvc/llvm-project?view=revision&revision=184408

Clang 3.4 is not yet released, but there is a release candidate, and
there is a dashboard test of Clang trunk.
Merge topic 'cross-compiling-toolchain-variables' into next

89ba7b0 Update the Clang pattern to specify compiler options.