Move Discord Rich Presence behind a compile-time option

src/CMakeLists.txt

@@ -19,7 +19,7 @@ if(APPLE)
 endif()
 
 add_executable(86Box 86box.c config.c log.c random.c timer.c io.c acpi.c apm.c
-    dma.c ddma.c discord.c nmi.c pic.c pit.c pit_fast.c port_6x.c port_92.c ppi.c pci.c
+    dma.c ddma.c nmi.c pic.c pit.c pit_fast.c port_6x.c port_92.c ppi.c pci.c
     mca.c usb.c fifo8.c device.c nvr.c nvr_at.c nvr_ps2.c machine_status.c ini.c)
 
 if(CMAKE_SYSTEM_NAME MATCHES "Linux")
@@ -51,6 +51,11 @@ if(DEV_BRANCH)
     add_compile_definitions(DEV_BRANCH)
 endif()
 
+if(DISCORD)
+    add_compile_definitions(DISCORD)
+    target_sources(86Box PRIVATE discord.c)
+endif()
+
 if(VNC)
     find_package(LibVNCServer)
     if(LibVNCServer_FOUND)