Currently if two modules Require eachother recursitvely, pkg-config just errs
Too many open files... It's useful to support this. For example, when building
cairo on Linux systems, cairo.pc requires cairo-xlib.pc, and cairo-xlib.pc also
requires cairo.pc. I have to work around it, but would have been nice if it
Fixed in commit 6d6dd43e. pkg-config now keeps track of the dependency chain as it descends and will break it if the requested dependency is already in it.