Add extra aclocal config required for homebrew

On at least some homebrew systems you will encounter the following
error after a new install when running servo's configure:

    servo/src/pixman/configure: line 13682: PKG_PROG_PKG_CONFIG: command not found
    servo/src/pixman/configure: line 13756: syntax error near unexpected token `GTK,'
    servo/src/pixman/configure: line 13756: `   PKG_CHECK_MODULES(GTK, gtk+-2.0 pixman-1)'

This appears to be related to mxcl/homebrew#5117 and mxcl/homebrew#3209.

Add a note to the README.md about how to resolve this issue with some
manual configuration.