tools: flex: fix build with automake 1.15.1

tools: flex: fix build with automake 1.15.1

After the update of GNU automake, the flex configure fails with

    configure.ac:31: error: version mismatch.  This is Automake 1.15.1,
    configure.ac:31: but the definition used by this AM_INIT_AUTOMAKE
    configure.ac:31: comes from Automake 1.15.

Apply the autoreconf host fixup to properly regenerate configure before
to avoid triggering the version mismatch. Also refresh the patch while
we're at it.

Fixes FS#885.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>