gl_MANYWARN_ALL_GCC() leads to many spurious warnings

[Samuel Bronson]

It seems that gl_MANYWARN_ALL_GCC is intended to enable (virtually)  
all warnings implemented by GCC, no matter how noisy or badly tested  
they are in whatever GCC version is in use.

The result is that, even after we explicitly disable dozens of them  
again, we still generally get a lot of spurious warnings when building  
with --enable-gcc-warnings.   Furthermore, since that option also  
enables -Werror, Emacs generally does not build at all in this  
configuration (unless you monkey around with WERROR_CFLAGS).

Why not simply use "-W -Wall -Wextra" and perhaps a few others, plus a  
few overrides to turn off the warnings those enable that aren't useful  
with Emacs?  That way, we'd only get warnings that had been reasonably  
well tested.