build: automake 1.12 to deal with flex & bison

build: automake 1.12 to deal with flex & bison

bison conditionally writes its output to different files based on the
filenames specified in the source code.  This could be disabled,
however...  flex changes its output filenames when "prefix" is
specified.  And ylwrap from <1.11 doesn't understand how to handle the
header file...

...so this requires automake 1.12 which can deal with this properly.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>