manywarnings: update the list of "all" warnings

manywarnings: update the list of "all" warnings

* m4/manywarnings.m4: Unite lists, and add many new options.
* build-aux/gcc-warning: New file.

Run this command with the latest gcc to see if they have added
options not yet on our list:

gl=.; comm -1 -3 \
  <(sed -n 's/^  *\(-[^ ]*\) .*/\1/p' $gl/m4/manywarnings.m4 |sort) \
  <(gcc --help=warnings|sed -n 's/^  \(-[^ ]*\) .*/\1/p' |sort \
    |grep -v --line-regexp -f <(cut -f1 $gl/build-aux/gcc-warning.spec))