[PHPC-2647] Out-of-source build: Don't necessarily generate flies in the source tree

[vector-of-bool]

This changeset modifies an AC_CONFIG_FILES directive to configure some of mongoc's files using relative paths. IIUC, using relative paths (instead of absolute ones) will make it read the file relative to the configure script, but write the file with the same name relative to the $PWD of the configure execution. This allows the configure script to leave the source tree more pristine.

To support this, the directories that are (possibly) generated by AC_CONFIG_FILES needs to be added as #include search paths. (The search directories are added based on $PWD, which feels somewhat brittle (is there a better autotools/autoconf variable to be used here? I couldn't seem to make it work otherwise.).)

(I'm not sure if there is an existing relevant Jira ticket for out-of-source build support tasks, so I created PHPC-2647 for this.)