Emacs release branch warnings with GCC 8

[Paul Eggert]

The emacs-26 branch has some warnings when compiled on Fedora 28, which 
is scheduled for release next week; see attached. This is because Fedora 
28 uses GCC 8.0.1, and when --enable-gcc-warnings is used (which is the 
default in a build from Git), GCC 8 issues some warnings that GCC 7 does 
not. I've patched the master to fix this problem in commits 
2b9ab8c8fba849da8bf2aa45e65b122bb937a6b3 and 
8c3215e7a47e3caaa005bf573765ed63e0739b89, and could backport these 
patches (or a subset) to emacs-26 if there's interest.

Only one of the warnings indicates a real bug, one in emacsclient that 
could cause undefined behavior. On the other hand, if we don't pacify 
GCC 8 now we're likely to get bug reports about the warnings for quite 
some time, and it might be less work for everybody involved if we squash 
the warnings now.

emacs-26.txt
Description: Text document