Don't use our version of ffs() if the system has one

configure.ac

@@ -180,7 +180,7 @@ dnl Checks for library functions.
 AC_FUNC_VPRINTF
 AC_CHECK_FUNCS([geteuid getuid link memmove memset mkstemp strchr strrchr \
 		strtol getopt getopt_long vsnprintf walkcontext backtrace \
-		getisax getzoneid shmctl64 strcasestr])
+		getisax getzoneid shmctl64 strcasestr ffs])
 AC_FUNC_ALLOCA
 dnl Old HAS_* names used in os/*.c.
 AC_CHECK_FUNC([getdtablesize],

dix/ffs.c

@@ -30,6 +30,8 @@ The Open Group.
 #include <dix-config.h>
 #endif
 
+#ifndef HAVE_FFS
+
 #include "dix.h"
 
 int
@@ -42,3 +44,5 @@ ffs(int i)
 	i >>= 1;
     return j;
 }
+
+#endif

include/dix-config.h.in

@@ -433,4 +433,7 @@
 /* Need the strcasestr function. */
 #undef NEED_STRCASESTR
 
+/* Define to 1 if you have the `ffs' function. */
+#undef HAVE_FFS
+
 #endif /* _DIX_CONFIG_H_ */