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/restful_server/restful_server.c

@@ -53,7 +53,7 @@ int main(int argc, char *argv[]) {
   int i;
   char *cp;
   const char *err_str;
-#ifdef MG_ENABLE_SSL
+#if MG_ENABLE_SSL
   const char *ssl_cert = NULL;
 #endif
 
@@ -75,7 +75,7 @@ int main(int argc, char *argv[]) {
       s_http_port = argv[++i];
     } else if (strcmp(argv[i], "-a") == 0 && i + 1 < argc) {
       s_http_server_opts.auth_domain = argv[++i];
-#ifdef MG_ENABLE_JAVASCRIPT
+#if MG_ENABLE_JAVASCRIPT
     } else if (strcmp(argv[i], "-j") == 0 && i + 1 < argc) {
       const char *init_file = argv[++i];
       mg_enable_javascript(&mgr, v7_create(), init_file);
@@ -86,11 +86,11 @@ int main(int argc, char *argv[]) {
       s_http_server_opts.per_directory_auth_file = argv[++i];
     } else if (strcmp(argv[i], "-r") == 0 && i + 1 < argc) {
       s_http_server_opts.url_rewrites = argv[++i];
-#ifndef MG_DISABLE_CGI
+#if !MG_DISABLE_CGI
     } else if (strcmp(argv[i], "-i") == 0 && i + 1 < argc) {
       s_http_server_opts.cgi_interpreter = argv[++i];
 #endif
-#ifdef MG_ENABLE_SSL
+#if MG_ENABLE_SSL
     } else if (strcmp(argv[i], "-s") == 0 && i + 1 < argc) {
       ssl_cert = argv[++i];
 #endif
@@ -103,7 +103,7 @@ int main(int argc, char *argv[]) {
   /* Set HTTP server options */
   memset(&bind_opts, 0, sizeof(bind_opts));
   bind_opts.error_string = &err_str;
-#ifdef MG_ENABLE_SSL
+#if MG_ENABLE_SSL
   if (ssl_cert != NULL) {
     bind_opts.ssl_cert = ssl_cert;
   }