build: require autoconf 2.69

It is only from version 2.69 that the 'AC_PROG_FC' macro has started to
set the '$GFC' variable to "yes" if the selected Fortran compiler is a
GNU compiler.  Since we use this variable in our configure.ac, we better
require autoconf 2.69 explicitly.

* configure.ac (AC_PRERQ): Require autoconf >= 2.69.
Remove a now-obsolete "FIXME" comment.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>