configury: re-factor to extract common code

Pull common code out of the provider .m4 scripts and back into a central .m4 file.  Now much of the setup and administration stuff for FI_<provider>_CONFIGURE is done in a central location -- the _CONFIGURE macros can just be small chunks of code that focus on checking to see if that provider can be built (e.g., check for headers and libraries and that's it).