configure: use AM_PROG_CC_C_O

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>

configure.ac

@@ -6,6 +6,7 @@ AX_PREFIX_CONFIG_H([rfb/rfbconfig.h])
 
 # Checks for programs.
 AC_PROG_CC
+AM_PROG_CC_C_O
 if test -z "$CC"; then
 	CCLD="\$(CC)"
 else