Fix non-C23-compatible prototypes.

--- units-2.24/parse.y	2024-11-08 21:47:09.000000000 +0000
+++ units-2.24/parse.y	2025-07-11 17:39:34.767021826 +0100
@@ -45,7 +45,8 @@
                               if (var) destroyunit(var); \
                               YYABORT; }
  
-int yylex();
+union UNITSSTYPE;
+int yylex(union UNITSSTYPE *lvalp, struct commtype *comm);
 void yyerror(struct commtype *comm, char *);
 
 #define MAXMEM 100
--- units-2.24/units.h	2024-11-15 02:24:47.000000000 +0000
+++ units-2.24/units.h	2025-07-11 17:33:15.111036898 +0100
@@ -50,13 +50,9 @@
    char *malloc(),  *realloc(),  *getenv();
 #endif
 
-#ifndef strchr
-#  ifdef NO_STRCHR
-#    define strchr(a,b) index((a),(b))
-#  else
-     char *strchr();
-#  endif
-#endif /* !strchr */
+#ifdef NO_STRCHR
+#  define strchr(a,b) index((a),(b))
+#endif
 
 #define E_NORMAL 0
 #define E_PARSE 1