Only use unsetenv() when it is available

AIX 4.3 dosn't support it, for example.

configure.ng

@@ -189,7 +189,7 @@ AC_CHECK_FUNCS([ \
 AC_CHECK_FUNCS_ONCE([ \
 	arc4random arc4random_stir gai_strerror getaddrinfo getnameinfo inet_aton
 	sigaction sigprocmask snprintf vsnprintf strdup strndup strlcpy strlcat \
-	strtok_r waitpid])
+	strtok_r unsetenv waitpid])
 
 # -- Configuration options --
 

src/ngircd/conn.c

@@ -152,7 +152,9 @@ my_sd_listen_fds(void)
 	if (!e || !*e)
 		return -1;
 	count = atoi(e);
+#ifdef HAVE_UNSETENV
 	unsetenv("LISTEN_FDS");
+#endif
 
 	return count;
 }