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 GCC optimization options [1], 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.

[1] https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html

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