PVS-Studio warning V814 for LibRaw

char *my_strcasestr (char *haystack, const char *needle)
{
  char *c;
  for (c = haystack; *c; c++)
    if (!strncasecmp(c, needle, strlen(needle)))
      return c;
  return 0;
}

This suspicious code was found in LibRaw project by PVS-Studio static code analyzer.
Warning message is:
V814 Decreased performance. The 'strlen' function was called multiple times inside the body of a loop. dcraw_common.cpp 72

PVS-Studio is a static analyzer for detecting bugs in the source code of applications written in C, C++, C++11, C++/CX. Site: http://www.viva64.com/en/pvs-studio/