Also, make it easier to maintain this in the future.
* build-aux/gcc-warning.spec: Add -Wabi-tag,
-Wconditionally-supported, -Wdelete-incomplete,
-Winherited-variadic-ctor, -Wvirtual-move-assign, -Wzerotrip.
Remove duplicates. Use tabs uniformly, as that's what 'cut' wants.
* m4/manywarnings.m4 (gl_MANYWARN_ALL_GCC): Add -Wdate-time,
-Wopenmp-simd. Use -fdiagnostics-show-option and -funit-at-a-time
only for older GCC versions that need them. Handke
-Wnormalized=nfc specially, so that the 'comm' command used
for maintenance doesn't get confused.