libbacktrace: avoid -Wpointer-arith errors

Based on patch from Kirill Müller.

	* configure.ac (ACX_PROG_CC_WARNING_OPTS): Add -Wpointer-arith.
	* pecoff.c (coff_add): Cast void pointers.
	* xcoff.c (xcoff_add): Likewise.
	* configure: Regenerate.

configure

@@ -12056,7 +12056,8 @@ save_CFLAGS="$CFLAGS"
 for real_option in -W -Wall -Wwrite-strings -Wstrict-prototypes \
 			  -Wmissing-prototypes -Wold-style-definition \
 			  -Wmissing-format-attribute -Wcast-qual \
-			  -Wno-attributes -Wno-unknown-attributes; do
+			  -Wno-attributes -Wno-unknown-attributes \
+			  -Wpointer-arith; do
   # Do the check with the no- prefix removed since gcc silently
   # accepts any -Wno-* option on purpose
   case $real_option in