Add header paths for system libraries as SYSTEM

This ensures correct include ordering and hides warnings coming
from unrelated code. Specifically, it fixes build on FreeBSD,
which otherwise fails due to to problem in lcms code:

    In file included from .../stratagus-3.3.1/src/action/action_defend.cpp:45:
    In file included from .../stratagus/work/stratagus-3.3.1/src/include/ui.h:56:
    In file included from .../stratagus-3.3.1/src/include/viewport.h:34:
    In file included from .../stratagus-3.3.1/src/include/fow.h:39:
    In file included from .../stratagus-3.3.1/src/include/video.h:228:
    In file included from /usr/local/include/libmng.h:385:
    In file included from /usr/local/include/libmng_types.h:181:
    /usr/local/include/lcms.h:1238:29: error: ISO C++17 does not allow 'register' storage class specifier [-Wregister]
    typedef int (* _cmsSAMPLER)(register WORD In[],
                                ^~~~~~~~~

CMakeLists.txt

@@ -816,7 +816,7 @@ endif()
 # Stratagus definitions
 
 add_definitions(-DUSE_ZLIB -DPIXMAPS=\"${PIXMAPSDIRABS}\")
-include_directories(${LUA_INCLUDE_DIR} ${SDL2_INCLUDE_DIR} ${SDL2_MIXER_INCLUDE_DIRS} ${SDL2_IMAGE_INCLUDE_DIRS} ${TOLUA++_INCLUDE_DIR} ${ZLIB_INCLUDE_DIR} ${PNG_INCLUDE_DIR})
+include_directories(SYSTEM ${LUA_INCLUDE_DIR} ${SDL2_INCLUDE_DIR} ${SDL2_MIXER_INCLUDE_DIRS} ${SDL2_IMAGE_INCLUDE_DIRS} ${TOLUA++_INCLUDE_DIR} ${ZLIB_INCLUDE_DIR} ${PNG_INCLUDE_DIR})
 set(stratagus_LIBS ${stratagus_LIBS} ${LUA_LIBRARIES} ${SDL2_LIBRARY} ${SDL2_MIXER_LIBRARIES} ${SDL2_IMAGE_LIBRARIES} ${TOLUA++_LIBRARY} ${ZLIB_LIBRARIES} ${PNG_LIBRARY})
 
 if(WIN32 AND NOT ENABLE_STDIO_REDIRECT)
@@ -825,26 +825,26 @@ endif()
 
 if(WITH_BZIP2 AND BZIP2_FOUND)
 	add_definitions(-DUSE_BZ2LIB ${BZIP2_DEFINITIONS})
-	include_directories(${BZIP2_INCLUDE_DIR})
+	include_directories(SYSTEM ${BZIP2_INCLUDE_DIR})
 	set(stratagus_LIBS ${stratagus_LIBS} ${BZIP2_LIBRARIES})
 endif()
 
 if(WITH_MNG AND MNG_FOUND)
 	add_definitions(-DUSE_MNG)
-	include_directories(${MNG_INCLUDE_DIR})
+	include_directories(SYSTEM ${MNG_INCLUDE_DIR})
 	add_definitions(${MNG_DEFINITIONS})
 	set(stratagus_LIBS ${stratagus_LIBS} ${MNG_LIBRARY})
 endif()
 
 if(WITH_OGGVORBIS AND OGGVORBIS_FOUND)
 	add_definitions(-DUSE_VORBIS)
-	include_directories(${OGGVORBIS_INCLUDE_DIR})
+	include_directories(SYSTEM ${OGGVORBIS_INCLUDE_DIR})
 	set(stratagus_LIBS ${stratagus_LIBS} ${OGG_LIBRARY} ${VORBIS_LIBRARY})
 endif()
 
 if(WITH_THEORA AND THEORA_FOUND)
 	add_definitions(-DUSE_THEORA)
-	include_directories(${THEORA_INCLUDE_DIR})
+	include_directories(SYSTEM ${THEORA_INCLUDE_DIR})
 	set(stratagus_LIBS ${stratagus_LIBS} ${THEORA_LIBRARY})
 endif()