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

docs/c-api/net.h/struct_mg_bind_opts.md

@@ -7,7 +7,7 @@ signature: |
     void *user_data;           /* Initial value for connection's user_data */
     unsigned int flags;        /* Extra connection flags */
     const char **error_string; /* Placeholder for the error string */
-  #ifdef MG_ENABLE_SSL
+  #if MG_ENABLE_SSL
     /* SSL settings. */
     const char *ssl_cert;    /* Server certificate to present to clients */
     const char *ssl_key;     /* Private key corresponding to the certificate.

docs/c-api/net.h/struct_mg_connect_opts.md

@@ -7,7 +7,7 @@ signature: |
     void *user_data;           /* Initial value for connection's user_data */
     unsigned int flags;        /* Extra connection flags */
     const char **error_string; /* Placeholder for the error string */
-  #ifdef MG_ENABLE_SSL
+  #if MG_ENABLE_SSL
     /* SSL settings. */
     const char *ssl_cert;    /* Client certificate to present to the server */
     const char *ssl_key;     /* Private key corresponding to the certificate.

docs/c-api/net.h/struct_mg_connection.md

@@ -14,7 +14,7 @@ signature: |
     size_t recv_mbuf_limit;  /* Max size of recv buffer */
     struct mbuf recv_mbuf;   /* Received data */
     struct mbuf send_mbuf;   /* Data scheduled for sending */
-  #if defined(MG_ENABLE_SSL)
+  #if MG_ENABLE_SSL
   #if !defined(MG_SOCKET_SIMPLELINK)
     SSL *ssl;
     SSL_CTX *ssl_ctx;

docs/c-api/net.h/struct_mg_mgr.md

@@ -6,12 +6,12 @@ signature: |
   struct mg_mgr {
     struct mg_connection *active_connections;
     const char *hexdump_file; /* Debug hexdump file path */
-  #ifndef MG_DISABLE_SOCKETPAIR
+  #if !MG_DISABLE_SOCKETPAIR
     sock_t ctl[2]; /* Socketpair for mg_broadcast() */
   #endif
     void *user_data; /* User data */
     void *mgr_data;  /* Implementation-specific event manager's data. */
-  #ifdef MG_ENABLE_JAVASCRIPT
+  #if MG_ENABLE_JAVASCRIPT
     struct v7 *v7;
   #endif
   };