Policy re -pedantic

[Zack Weinberg]

There are two possible interpretations of -pedantic for the C compiler:

- Always be pedantic about C89.
- Be pedantic about whichever standard version has been selected with -std=.

I believe that currently we implement #1.  I think #2 makes more sense,
since the list of things to be pedantic about is different: for example,
long long constants are a GCC extension to C89, but they are standard in
C9x.

zw