remove various preprocessor conditionals

- HAVE_IEEEFP_H is nowhere defined or needed at all
- FreeBSD < 4 is unsupported since years
- MSVC workaround (around what exactly?)

src/string.c

@@ -5,12 +5,7 @@
 */
 
 #include <ctype.h>
-#ifndef SIZE_MAX
- /* Some versions of VC++
-  * has SIZE_MAX in stdint.h
-  */
-# include <limits.h>
-#endif
+#include <limits.h>
 #include <stddef.h>
 #include <stdlib.h>
 #include <string.h>