c: port to HP-UX 11.23

Problem reported by Albert Chin in:
https://lists.gnu.org/r/bug-bison/2021-01/msg00029.html
* data/skeletons/c.m4 (b4_c99_int_type_define):
Work around HP-UX bug.

NEWS

@@ -2,6 +2,9 @@ GNU Bison NEWS
 
 * Noteworthy changes in release ?.? (????-??-??) [?]
 
+** Bug fixes
+
+*** C-language parsers now work around quirks of HP-UX 11.23 (2003).
 
 * Noteworthy changes in release 3.7.4 (2020-11-14) [stable]
 

data/skeletons/c.m4

@@ -242,6 +242,18 @@ typedef int_least16_t yytype_int16;
 typedef short yytype_int16;
 #endif
 
+/* Work around bug in HP-UX 11.23, which defines these macros
+   incorrectly for preprocessor constants.  This workaround can likely
+   be removed in 2023, as HPE has promised support for HP-UX 11.23
+   (aka HP-UX 11i v2) only through the end of 2022; see Table 2 of
+   <https://h20195.www2.hpe.com/V2/getpdf.aspx/4AA4-7673ENW.pdf>.  */
+#ifdef __hpux
+# undef UINT_LEAST8_MAX
+# undef UINT_LEAST16_MAX
+# define UINT_LEAST8_MAX 255
+# define UINT_LEAST16_MAX 65535
+#endif
+
 #if defined __UINT_LEAST8_MAX__ && __UINT_LEAST8_MAX__ <= __INT_MAX__
 typedef __UINT_LEAST8_TYPE__ yytype_uint8;
 #elif (!defined __UINT_LEAST8_MAX__ && defined YY_STDINT_H \