Change from using #ifdef to #if for features tests

"#if FOO" still works with simple -DFOO, but gives more flexibility.
Specifically, if user expressed no preference (FOO is not defined),
we can apply reasonable defaults (this is the legitimate use of ifdef).

In short, from now on, please use

 #if MG_ENABLE_FOO

instead of

 #ifdef MG_ENABLE_FOO

Since we are all used to #ifdef, this change also adds a precommit check
to police this. Specifically, in *.h and *.c files that are Copyright Cesanta,
"ifdef" and "if defined()" are not allowed to be used with macros that contain
ENABLE or DISABLE, unless the like also contains "ifdef-ok".

Hence, if you are sure you want to use ifdef, use this:

 #ifdef MG_ENABLE_FOO /* ifdef-ok */

PUBLISHED_FROM=9be829448f53cff575d6cae8b9945fb12531c15a

examples/load_balancer/load_balancer.c

@@ -60,7 +60,7 @@ static int s_num_vhost_backends = 0, s_num_default_backends = 0;
 static int s_sig_num = 0;
 static int s_backend_keepalive = 0;
 static FILE *s_log_file = NULL;
-#ifdef MG_ENABLE_SSL
+#if MG_ENABLE_SSL
 const char *s_ssl_cert = NULL;
 #endif
 
@@ -206,7 +206,7 @@ static void forward(struct conn_data *conn, struct http_message *hm,
   for (i = 0; i < MG_MAX_HTTP_HEADERS && hm->header_names[i].len > 0; i++) {
     struct mg_str hn = hm->header_names[i];
     struct mg_str hv = hm->header_values[i];
-#ifdef MG_ENABLE_SSL
+#if MG_ENABLE_SSL
     /*
      * If we terminate SSL and backend redirects to local HTTP port,
      * strip protocol to let client use HTTPS.
@@ -598,7 +598,7 @@ int main(int argc, char *argv[]) {
       vhost = NULL;
       redirect = 0;
       i += 2;
-#ifdef MG_ENABLE_SSL
+#if MG_ENABLE_SSL
     } else if (strcmp(argv[i], "-s") == 0 && i + 1 < argc) {
       s_ssl_cert = argv[++i];
 #endif