Why does Apache want to use xcode for installation resources?
It's hard to really know why the authors of the tool made a specific choice without tracking them down in many cases, but this one shouldn't be hard to guess.

Xcode is how Apple distributes a working C compiler to each version of OS X. therefore checking in Xcode for gcc makes good sense to me. 

As you correctly have demonstrated, if you prefer to install another compiler or store is elsewhere, the configure tool is happy to use another path to the compiler.