Disable userfaultfd again by making it opt-in (#1777)

* Disable userfaultfd again

* Let userfd be be opt-in instead of disabled

Author: C0rn3j

CMakeLists.txt

@@ -890,7 +890,8 @@ if (ENABLE_DISCORD_RPC)
     target_compile_definitions(shadps4 PRIVATE ENABLE_DISCORD_RPC)
 endif()
 
-if (${CMAKE_SYSTEM_NAME} STREQUAL "Linux")
+# Optional due to https://github.com/shadps4-emu/shadPS4/issues/1704
+if (${CMAKE_SYSTEM_NAME} STREQUAL "Linux" AND ENABLE_USERFAULTFD)
     target_compile_definitions(shadps4 PRIVATE ENABLE_USERFAULTFD)
 endif()