configure: check for program_invocation_name

For portabiliy, check for program_invocation_name during configure and
define HAVE_PROGRAM_INVOCATION_NAME accordingly. Use of this symbol is
now enclosed with the appropriate #ifdef block.

The symbol program_invocation_name is only used for error message
handling using error(), so it's safe to omit this if it is not
available.

sysctl.c

@@ -675,7 +675,9 @@ int main(int argc, char *argv[])
 		{NULL, 0, NULL, 0}
 	};
 
+#ifdef HAVE_PROGRAM_INVOCATION_NAME
 	program_invocation_name = program_invocation_short_name;
+#endif
 	setlocale(LC_ALL, "");
 	bindtextdomain(PACKAGE, LOCALEDIR);
 	textdomain(PACKAGE);