Fix link-time optimization to work correctly for unit tests (i.e. -fl…

…to flag) (#1290)

* We compile various c files into object and package them into library
  (.a file) using ar to feed to unit tests. With new GCC versions, the
  objects inside such library don't participate in LTO process without
  additional flags.
* Here is a direct quote from gcc documentation explaining this issue:
  "If you are not using a linker with plugin support and/or do not
  enable the linker plugin, then the objects inside libfoo.a are extracted
  and linked as usual, but they do not participate in the LTO optimization
  process. In order to make a static library suitable for both LTO
  optimization and usual linkage, compile its object files with -flto-ffat-lto-objects."
* Read full documentation about -flto at https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html
* Without this additional flag, I get following errors while executing
  "make test-unit". With this change, those errors go away.

```
ARCHIVE libvalkey.a
ar: threads_mngr.o: plugin needed to handle lto object
...
..
.
/tmp/ccDYbMXL.ltrans0.ltrans.o: In function `dictClear':
/local/workplace/elasticache/valkey/src/unit/../dict.c:776: undefined
reference to `valkey_free'
/local/workplace/elasticache/valkey/src/unit/../dict.c:770: undefined
reference to `valkey_free'
/tmp/ccDYbMXL.ltrans0.ltrans.o: In function `dictGetVal':
```

Signed-off-by: Parth Patel <[email protected]>

src/Makefile

@@ -27,6 +27,7 @@ ifeq ($(OPTIMIZATION),-O3)
 	else
 		OPTIMIZATION+=-flto=auto
 	endif
+	OPTIMIZATION += -ffat-lto-objects
 endif
 ifneq ($(OPTIMIZATION),-O0)
 	OPTIMIZATION+=-fno-omit-frame-pointer