Add multithreading option for CMake

src/CMakeLists.txt

@@ -231,6 +231,8 @@ set(SRB2_CONFIG_HAVE_OPENMPT ON CACHE BOOL
 	"Enable OpenMPT support.")
 set(SRB2_CONFIG_HAVE_CURL ON CACHE BOOL
 	"Enable curl support, used for downloading files via HTTP.")
+set(SRB2_CONFIG_HAVE_THREADS ON CACHE BOOL
+	"Enable multithreading support.")
 if(${CMAKE_SYSTEM} MATCHES Windows)
 	set(SRB2_CONFIG_HAVE_MIXERX ON CACHE BOOL
 		"Enable SDL Mixer X support.")

src/sdl/CMakeLists.txt

@@ -55,6 +55,12 @@ set(SRB2_SDL2_HEADERS
 	sdlmain.h
 )
 
+if(${SRB2_CONFIG_HAVE_THREADS})
+	set(SRB2_HAVE_THREADS ON)
+	set(SRB2_SDL2_SOURCES ${SRB2_SDL2_SOURCES} i_threads.c)
+	add_definitions(-DHAVE_THREADS)
+endif()
+
 source_group("Interface Code" FILES ${SRB2_SDL2_SOURCES} ${SRB2_SDL2_HEADERS})
 
 # Dependency