Fix LTO warning by enabling parallel compilation

The previous use of -flto caused a warning:
"lto-wrapper: warning: using serial compilation of 2 LTRANS jobs"

To resolve this and enable parallel compilation, -flto=auto is now used
This option leverages GNU make's job server when available and falls
back to autodetection of CPU threads.

Makefile

@@ -32,7 +32,7 @@ CFLAGS += -flto
 endif
 endif
 ifeq ("$(CC_IS_GCC)", "1")
-CFLAGS += -flto
+CFLAGS += -flto=auto
 endif
 ifeq ("$(CC_IS_CLANG)", "1")
 CFLAGS += -flto=thin -fsplit-lto-unit