clang's build performance

[Akim Demaille]

I have also observed the regression in compilation speed.  Which
is not really surprising: GCC was slower, but producing better
code (at least on my project).  Now clang is catching up on
the quality of the produced code, but that does require more
compilation time.

Is there a means to specify configure flags for a specific port?
Since I'm spending quite some time compiling, I'd like to try
compiling clang-3.5 itself with -O3.