Yes, I suggest doing a check and then installing PETSc based on what's enabled.  The [HomeBrew Formula Cookbook](https://github.com/mxcl/homebrew/wiki/Formula-Cookbook) suggests you look at the graphviz example, which has the following relevant lines:

    option 'with-x', 'Build with X11 support'
    depends_on :x11 if build.include? 'with-x' or MacOS::X11.installed?
    args << "--without-x" unless build.include? 'with-x' or MacOS::X11.installed?

In PETSc, you could directly use the first two lines, and the equivalent configure flag would be:

    args << "--with-x=0" unless build.include? 'with-x' or MacOS::X11.installed?

You could do something similar if you wanted to make a gfortran compiler available.

Sorry to be so nitpicky, it's a great thing to see people actively interested in getting PETSc installed reliably on OS X machines, and I'd like to help make this formula bullet-proof :)