Fix libxml2 universal binary compilation on 10.6

 [libxml2] Add "--disable-dependency-tracking" configure option to fix build issues on 10.6 with universal binaries.

Without this patch I get this error:

gcc-4.2: -E, -S, -save-temps and -M options are not allowed with multiple -arch flags

when trying to "brew install --universal libxml2" on 10.6.

Technically the --disable-dependency-tracking is not needed when not
doing universal binaries, but dependency tracking only matters during
development and

    system "./configure", "--prefix=#{prefix}", "--without-python", *build.universal? ? "--disable-dependency-tracking" : []

seemed overcomplicated.