make: limit changes to pragmas

The special target .PRAGMA could be used to set or reset pragmas.
Doing anything other than setting pragmas very early in execution
is likely to be problematic.

Limit the abilities of .PRAGMA:

- Specifying .PRAGMA with no prerequisites now does nothing:
  pragmas are not reset.

- The posix_2017 and posix_202x pragmas can only be used to change
  the enforced POSIX level from the default.  Any further attempt
  to change POSIX level results in a warning.

Adds 16-32 bytes.

miscutils/make.c

@@ -656,7 +656,10 @@ set_pragma(const char *name)
 		if (idx >= BIT_POSIX_2017) {
 			// POSIX level is stored in a separate variable.
 			// No bits in 'pragma' are used.
-			posix_level = idx - BIT_POSIX_2017;
+			if (posix_level == DEFAULT_POSIX_LEVEL)
+				posix_level = idx - BIT_POSIX_2017;
+			else if (posix_level != idx - BIT_POSIX_2017)
+				warning("unable to change POSIX level");
 		} else {
 			pragma |= 1 << idx;
 		}
@@ -693,10 +696,6 @@ addrule(struct name *np, struct depend *dp, struct cmd *cp, int flag)
 	if ((np->n_flag & N_SPECIAL) && !dp && !cp) {
 		if (strcmp(np->n_name, ".PHONY") == 0)
 			return;
-#if ENABLE_FEATURE_MAKE_POSIX
-		if (strcmp(np->n_name, ".PRAGMA") == 0)
-			pragma = 0;
-#endif
 		freerules(np->n_rule);
 		np->n_rule = NULL;
 		return;