configure.ac: Add the check for the ffs function
configure.ac: Add the check for the ffs function

When Cogl was sharing Clutter's configure script, it had a check for
the ffs function of libc so that it can provide a fallback if it is
not available. This fallback was missed in the split out so Cogl would
end up always using the fallback.

collects/cogl,Distrotech/cogl,Distrotech/cogl,djdeath/cogl-android,djdeath/cogl,spatulasnout/cogl,spatulasnout/cogl,gcampax/cogl,gcampax/cogl,djdeath/cogl,gcampax/cogl,gcampax/cogl,djdeath/cogl-android,djdeath/cogl,djdeath/cogl-android,collects/cogl,Distrotech/cogl,Distrotech/cogl