Assume STDC_HEADERS will always be defined

We're in 2021.  C89 is everywhere; in fact, there are many other
assumptions in the code that wouldn't probably hold on
pre-standard C environments.  Let's simplify and assume that C89
is available.

The specific assumptions are that:
- <string.h>, and <stdlib.h> are available
- strchr(3), strrchr(3), and strtok(3) are available
- isalpha(3), isspace(3), isdigit(3), and isupper(3) are available

I think we can safely assume we have all of those.

Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>

lib/defines.h

@@ -61,16 +61,8 @@ extern char * textdomain (const char * domainname);
 #endif
 #endif
 
-#if STDC_HEADERS
-# include <stdlib.h>
-# include <string.h>
-#else				/* not STDC_HEADERS */
-# ifndef HAVE_STRCHR
-#  define strchr index
-#  define strrchr rindex
-# endif
-char *strchr (), *strrchr (), *strtok ();
-#endif				/* not STDC_HEADERS */
+#include <stdlib.h>
+#include <string.h>
 
 #if HAVE_ERRNO_H
 # include <errno.h>