I attempted to compile flom on an old Debian Lenny box. configure found the library, but didn't check that it was new enough, so that the eventual compiler failed:
/tmp/flom-0.5.0/src/flom_locker.c:68: undefined reference to `g_ptr_array_new_with_free_func'
I notice that configure.ac uses pkg-config directly, rather than pkg-config's autoconf macro (pkg.m4), which allows you to specify a minimum version for the required libraries. pkg.m4 is distributed with pkg-config.
Please consider using pkg.m4 and specifying a minimum required glib2.