Port --enable-gcc-warnings to GCC 8

Port --enable-gcc-warnings to GCC 8

* configure.ac: Do not use GCC 8’s new -Wcast-align flag.
* lib-src/ebrowse.c (xmalloc):
* lib-src/emacsclient.c (xmalloc, xstrdup):
* lib-src/etags.c (xmalloc):
* lib-src/make-docfile.c (xmalloc):
* lib-src/movemail.c (xmalloc):
* src/dispnew.c (new_glyph_pool):
* src/regex.c (xmalloc):
* src/term.c (tty_menu_create):
* src/tparam.h (tparam):
Use ATTRIBUTE_MALLOC.  Also see GCC bug 85562.
* lib-src/emacsclient.c (fail):
Do not dereference a null pointer.
* src/frame.c (delete_frame):
Add a decl with UNINIT to work around GCC bug 85563.
* src/menu.h (finish_menu_items):
Do not use attribute const.
* src/regex.c (analyze_first): Use FALLTHROUGH, not a comment.