Fix extern symbols defined and not used

Fix extern symbols defined and not used

* src/alloc.c: Always include <signal.h>.
(malloc_warning) [!SIGDANGER && (SYSTEM_MALLOC || HYBRID_MALLOC)]:
Do not define; unused.
* src/emacs.c, src/lisp.h (might_dump) [!DOUG_LEA_MALLOC]: Now static.
* src/gmalloc.c (gdefault_morecore): Rename from __default_morecore,
to avoid collision with glibc.  Now static.  All uses changed.
* src/lastfile.c (my_edata): Define only if
((!defined SYSTEM_MALLOC && !defined HYBRID_MALLOC && !defined
WINDOWSNT) \ || defined CYGWIN || defined DARWIN_OS).
(Bug#22086)