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>

configure.ac

@@ -38,7 +38,6 @@ dnl Checks for libraries.
 
 dnl Checks for header files.
 AC_HEADER_DIRENT
-AC_HEADER_STDC
 AC_HEADER_SYS_WAIT
 AC_HEADER_STDBOOL
 

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 */
 
 #if HAVE_ERRNO_H
 # include <errno.h>

libmisc/getdate.y

@@ -31,11 +31,7 @@
 #include <ctype.h>
 #include <time.h>
 
-#if defined (STDC_HEADERS) || (!defined (isascii) && !defined (HAVE_ISASCII))
 #define IN_CTYPE_DOMAIN(c) 1
-#else
-# define IN_CTYPE_DOMAIN(c) isascii(c)
-#endif
 
 #define ISSPACE(c) (IN_CTYPE_DOMAIN (c) && isspace (c))
 #define ISALPHA(c) (IN_CTYPE_DOMAIN (c) && isalpha (c))
@@ -54,9 +50,7 @@
 
 #include "getdate.h"
 
-#if defined (STDC_HEADERS)
 #include <string.h>
-#endif
 
 /* Some old versions of bison generate parsers that use bcopy.
    That loses on systems that don't provide the function, so we have