[Dovecot] fallocate and glibc 2.10

In glibc 2.10 (32 bit) fallocate() exists but fallocate64() doesn't. When _FILE_OFFSET_BITS==64, fallocate() is a redirect to fallocate64() and the program can't be linked (fails to find symbol fallocate64). See http://bugzilla.redhat.com/500487

Attached patch detects fallocate() more robustly to guard against this problem.

Cheers, Paul.