Fix LTO warning by enabling parallel compilation

When using make for builds, the following warning is observed:
lto-wrapper: warning: using serial compilation of 2 LTRANS jobs.

To address this, -flto=auto is now used.
According to the [GCC documentation](https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html):
Use -flto=auto to use GNU make’s job server, if available, or otherwise
fall back to autodetection of the number of CPU threads present in your
system.

While this patch does not fully suppress the warning in certain GCC
versions (11.4 or later but below 12), it improves build performance by
enabling parallel LTO compilation whenever possible.

Author: eleanorLYJ

Makefile

@@ -48,7 +48,7 @@ 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