test for pread and use it instead of lseek+read in cdb and io_mmapwritefile

cdb/cdb.c

@@ -6,6 +6,7 @@
 #include <errno.h>
 #include "byte.h"
 #include "cdb.h"
+#include "havepread.h"
 #ifdef __MINGW32__
 #include "windows.h"
 #else
@@ -63,16 +64,25 @@ int cdb_read(struct cdb *c,unsigned char *buf,unsigned long len,uint32 pos) {
     byte_copy(buf,len,c->map + pos);
   }
   else {
+#ifndef HAVE_PREAD
     if (lseek(c->fd,pos,SEEK_SET) == -1) return -1;
+#endif
     while (len > 0) {
-      int r;
+      ssize_t r;
       do
+#ifdef HAVE_PREAD
+	r = pread(c->fd,buf,len,pos);
+#else
         r = read(c->fd,buf,len);
+#endif
       while ((r == -1) && (errno == EINTR));
       if (r == -1) return -1;
       if (r == 0) goto FORMAT;
       buf += r;
       len -= r;
+#ifdef HAVE_PREAD
+      pos += r;
+#endif
     }
   }
   return 0;