Skip C build if not changed - step 1

Note: this is only the first step; the C sources won't be built until
the second step is done.

The actual build takes about 1.3s on cloud2 which still runs the docker
containers and image layers on the very slow spinning disk. cloud3 has
docker mounted on the SSD disk; measured speedups of 20-30%

PUBLISHED_FROM=c5acef1d64ebc8dca8ed31f51685c8b3090c5767

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

@@ -6,10 +6,10 @@ signature: |
   void mg_mgr_init(struct mg_mgr *mgr, void *user_data);
 ---
 
-Initialise Mongoose manager. Side effect: ignores SIGPIPE signal.
-`mgr->user_data` field will be initialised with a `user_data` parameter.
-That is an arbitrary pointer where the user code can associate some data
+Initialize Mongoose manager. Side effect: ignores SIGPIPE signal.
+`mgr->user_data` field will be initialized with `user_data` parameter.
+That is an arbitrary pointer, where user code can associate some data
 with the particular Mongoose manager. For example, a C++ wrapper class
-could be written in which case `user_data` can hold a pointer to the
+could be written, in which case `user_data` can hold a pointer to the
 class instance.