include no SDL headers if no headless build

=3.6.1

@@ -0,0 +1,4 @@
+Collecting protobuf
+ Using cached protobuf-4.21.12-cp37-abi3-manylinux2014_x86_64.whl (409 kB)
+Installing collected packages: protobuf
+Successfully installed protobuf-4.21.12

src/CMakeLists.txt

@@ -36,6 +36,9 @@ set(CMAKE_BUILD_TYPE "${CMAKE_BUILD_TYPE}" CACHE STRING
 # Set some random things default to not being visible in the GUI
 mark_as_advanced(EXECUTABLE_OUTPUT_PATH LIBRARY_OUTPUT_PATH)
 
+if(BUILD_CLIENT AND BUILD_HEADLESS)
+ find_package(SDL2 REQUIRED)
+endif()
 
 if(NOT (BUILD_CLIENT OR BUILD_SERVER))
  message(WARNING "Neither BUILD_CLIENT nor BUILD_SERVER is set! Setting BUILD_SERVER=true")
@@ -64,7 +67,6 @@ if(NOT USE_CURL)
  endif()
 endif()
 
-
 option(ENABLE_GETTEXT "Use GetText for internationalization" ${BUILD_CLIENT})
 set(USE_GETTEXT FALSE)
 
@@ -567,6 +569,7 @@ if(BUILD_CLIENT)
  ${PROJECT_NAME}
  ${ZLIB_LIBRARIES}
  IrrlichtMt::IrrlichtMt
+ $<$<BOOL:BUILD_HEADLESS>:SDL2::SDL2>
  ${ZSTD_LIBRARY}
  ${ZMQ_LIBRARY}
  ${ZMQPP_LIBRARY}

src/client/renderingengine.cpp

@@ -35,7 +35,9 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include "inputhandler.h"
 #include "gettext.h"
 #include "../gui/guiSkin.h"
-#include <SDL_video.h>
+#ifdef BUILD_HEADLESS
+ #include <SDL_video.h>
+#endif
 
 #if !defined(_WIN32) && !defined(__APPLE__) && !defined(__ANDROID__) && \
  !defined(SERVER) && !defined(__HAIKU__)