Disable FORTIFY_SOURCE when compiling without optims

Otherwise the build is cluttered with

```
/nix/store/fwpn2f7a4iqszyydw7ag61zlnp6xk5d3-glibc-2.30-dev/include/features.h:382:4: warning: #warning _FORTIFY_SOURCE requires compiling with optimization (-O) [-Wcpp]
  382 | #  warning _FORTIFY_SOURCE requires compiling with optimization (-O)
      |    ^~~~~~~
```

when building with `OPTIMIZE=0`

Makefile

@@ -26,7 +26,7 @@ OPTIMIZE = 1
 ifeq ($(OPTIMIZE), 1)
   GLOBAL_CXXFLAGS += -O3
 else
-  GLOBAL_CXXFLAGS += -O0
+  GLOBAL_CXXFLAGS += -O0 -U_FORTIFY_SOURCE
 endif
 
 include mk/lib.mk