Why doesn't OS X 10.8 Mountain Lion find X11 libraries when building software?
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So, we all know that Mountain Lion doesn't ship with X11 anymore and users needing X11 are directed to download Xquartz. Xquartz installs to `/opt`, but it also symlinks `X11` and `X11R6` to `/usr`. But when building software that requires linking to X11 include files, I've discovered that I must pass an environment variable adding `/usr/X11/include` (or `/opt/X11/include`) to the library search path to get `./configure` to find the X11 libraries. My question is *why?*

I've done some research on Google (many results pointing back to Stack Overflow), and I've read Apple's documentation, and these sources all indicate that there is no equivalent in OS X to the `/etc/ld.so.conf` file found in many (if not all) Linux distributions. Apple even states that `DYLD_LIBRARY_PATH` is empty by default. However, under Lion (with Apple's last 'official' X11 installed), the same `./configure` scripts would find the X11 libraries without adding anything to the library search path. 

So, *why* can't `./configure` scripts find X11 libraries in Mountain Lion without explicit modification of the library search path?