cmake: use -flto=auto compiler flag when supported

Use -flto=auto compiler flag when supported, this silence this
GCC warning:

> lto-wrapper: warning: using serial compilation of # LTRANS jobs

This also greatly speeds-up the linkage time as it enables LTO
multithreading in GCC (either by using Make jobserver if detected,
either by detecting CPU cores).

Author: illwieckz

cmake/DaemonFlags.cmake

@@ -423,9 +423,13 @@ else()
 	# Saigo NaCl compiler doesn't support LTO, the flag is accepted but linking fails
 	# with “unable to pass LLVM bit-code files to linker” error.
 	if (USE_LTO AND NOT NACL)
-		try_c_cxx_flag(LTO "-flto")
+		try_c_cxx_flag(LTO_AUTO "-flto=auto")
 
-		if (FLAG_LTO)
+		if (NOT FLAG_LTO_AUTO)
+			try_c_cxx_flag(LTO "-flto")
+		endif()
+
+		if (FLAG_LTO_AUTO OR FLAG_LTO)
 			# Pass all compile flags to the linker.
 			set_linker_flag("${CMAKE_CXX_FLAGS}")