Fix trigger_once (#3)

* Make project compile on Linux

- Updated CMake files to support build on Linux
- Deleted original Linux makefiles as they are not needed since CMake is introduced
- Fixed minor problems in the code which prevented the project from compiling

It likely breaks the Windows build, but hopefully it only causes minor problems
those are easy to fix in a way that allows the project to compile on both platforms.

The Linux CMake files don't try to deploy the built libraries to the game mod
directory (people likely build in a separate environment, not where they have the
game installed).

* Explicitly link SDL2 to client library

- Added SDL2 to the list of linked libraries to client.so
Not sure if it helps anything, the library loaded without it successfully,
but it doesn't hurt either.

- Fixed symlinks to Linux and MacOS builds of SDL2
They were supposed to be symlinks and they are in Valve's repo, but in
Solokiller's fork they lost their symlink flag with this commit:
twhl-community/halflife-updated@6509102

* Port fix for getting mouse stuck in box

Apparently, the problem is caused by this issue:
ValveSoftware/halflife#1377

Which Solokiller has a fix for:
twhl-community/halflife-updated@92ffa23

But seems to be originated from L453rh4wk:
ValveSoftware/halflife#1546 (comment)

* Add CTriggerOnce.cpp to server library

Co-authored-by: MegaBrutal <megabrutal@devcentre>

dlls/CMakeLists.txt

@@ -201,6 +201,7 @@ set(TRIGGER_SRC
 	triggers/CTriggerHurt.cpp
 	triggers/CTriggerMonsterJump.cpp
 	triggers/CTriggerMultiple.cpp
+	triggers/CTriggerOnce.cpp
 	triggers/CTriggerPush.cpp
 	triggers/CTriggerRelay.cpp
 	triggers/CTriggerSave.cpp

dlls/triggers/CTriggerOnce.cpp

@@ -1,7 +1,6 @@
 #include "extdll.h"
 #include "util.h"
 #include "cbase.h"
-#include "player.h"
 #include "saverestore.h"
 #include "trains.h"			// trigger_camera has train functionality
 #include "gamerules.h"