Really use Unreal sysroot

CMake/LinuxToolchain.cmake

@@ -59,6 +59,12 @@ set (
 	CACHE PATH ""
 )
 
+set(
+	CMAKE_SYSROOT
+	${UE_SYSROOT}
+	CACHE FILEPATH ""
+)
+
 set (
 	UE_THIRD_PARTY
 	${UE_ROOT}/Engine/Source/ThirdParty CACHE PATH ""
@@ -85,13 +91,15 @@ set (
 )
 
 add_compile_options (
+	-fPIC
+	-pthread
 	-fms-extensions
 	-fno-math-errno
 	-fdiagnostics-absolute-paths
 	$<$<COMPILE_LANGUAGE:CXX>:-stdlib=libc++>
 )
 
-add_link_options (-stdlib=libc++ -L${UE_LIBS} )
+add_link_options (-stdlib=libc++ -L${UE_LIBS} -lpthread)
 
 set (
 	CMAKE_AR