add DFHACK_DISABLE env var

docs/Core.rst

@@ -336,6 +336,11 @@ on UNIX-like systems:
 
   DFHACK_SOME_VAR=1 ./dfhack
 
+- ``DFHACK_DISABLE``: if set, DFHack will not initialize, not even to redirect
+  :file:`stdout.txt` or :file:`stderr.txt`. This is provided as an alternative
+  to the ``--disable-dfhack`` commandline parameter above for when environment
+  variables are more convenient.
+
 - ``DFHACK_PORT``: the port to use for the RPC server (used by ``dfhack-run``
   and `remotefortressreader` among others) instead of the default ``5000``. As
   with the default, if this port cannot be used, the server is not started.

library/Hooks.cpp

@@ -8,6 +8,13 @@ static bool disabled = false;
 // called from the main thread before the simulation thread is started
 // and the main event loop is initiated
 DFhackCExport void dfhooks_init() {
+    if (getenv("DFHACK_DISABLE")) {
+        fprintf(stdout, "dfhack: DFHACK_DISABLE detected in environment; disabling\n");
+        disabled = true;
+        return;
+    }
+
+    // we need to init DF globals before we can check the commandline
     if (!DFHack::Core::getInstance().InitMainThread() || !df::global::game)
         return;
     const std::string & cmdline = df::global::game->command_line.original;