It is possible to define what architecture you wish to compile against on MacOS using the following C/C++ flag(s):
-arch ppc (ppc 32bit)
-arch ppc64 (ppc 64bit)
-arch i386 (intel 32bit)
-arch x86_64 (intel 64bit)
Its even possible to combine them to create "Universal Binaries" however the endian-ness of Firebird on ppc v's intel and the way that Firebird is currently built makes this somewhat impossible. Something for the future I think.