Fix all implicit declarations

AC_HEADER_TIME is only needed if you need to include time.h and
sys/time.h at the same time.

The truth is this codebase is an unholy mess but at least this fixes
the warnings.

ppmtoinfo.c

@@ -1,12 +1,10 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <time.h>
 #ifdef HAVE_UNISTD_H
 #include <unistd.h>
 #endif
-#ifdef HAVE_SYS_TIME_H
-#include <sys/time.h>
-#endif
 
 #define WB_DISKMAGIC	0xe310
 #define WB_DISKVERSION	1