Newer automakes error out due to per-file CFLAGS being used unless the
macro AM_PROG_CC_C_O is set in configure.ac.

[jes: The macro AM_PROG_CC_C_O has been around since 1999, so it should
 be safe.]
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>