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>