Test for existence of strerror.

SunOS 4.1 claims that it is __STDC__, but it does not have strerror
in string.h.  Instead of using __STDC__, this puts a direct test
for strerror in configure, and uses that information in gzguts.h.

gzguts.h

@@ -100,7 +100,7 @@
 #  include <windows.h>
 #  define zstrerror() gz_strwinerror((DWORD)GetLastError())
 #else
-#  ifdef STDC
+#  ifndef NO_STRERROR
 #    include <errno.h>
 #    define zstrerror() strerror(errno)
 #  else