Add FORTIFY and SANITIZE options for DEBUG builds

SANITIZE is not fully supported (or recommended) yet.

FORTIFY will include some performance hits, but those might be offset by -O1
(which is apparently required for _FORTIFY_SOURCE to have any effect).
Prevviosly, DEBUG builds used (implicit -O0).
I'm building and running sbbs on Vertrauen (git.synchro.net) now with FORTIFY.

Set FORTIFY=1 in your localdefs.mk if you wish to have this build option
enabled persistently.

src/build/Common.gmake

@@ -153,6 +153,13 @@ ifdef DEBUG
   CFLAGS	+=	-fprofile-arcs -ftest-coverage
   LDFLAGS	+=	-fprofile-arcs -ftest-coverage
  endif
+ ifdef FORTIFY
+  CFLAGS += -D_FORTIFY_SOURCE=3 -O1
+ endif
+ ifdef SANITIZE
+  CFLAGS += -fsanitize=undefined,address
+  LDFLAGS += -fsanitize=undefined,address
+ endif
 else
  ifdef USE_UPROFILE
   CFLAGS	+=	-fprofile-use