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"Some compilers offer an extension that allows ## to appear after a comma and
before __VA_ARGS__, in which case the ## does nothing when the variable
arguments are present, but removes the comma when the variable arguments are not
present: this makes it possible to define macros such as fprintf (stderr,
format, ##__VA_ARGS__)" [1]

This is problematic for Chromium since ##__VA_ARGS__ doesn't swallow the
preceding comma in standards-compliant mode on gcc [2].

Since we use this feature in Chromium (eg base), switch the gcc config back to
-std=gnu*.  C++20 adds a standards-compliant macro called __VA_OPT__ that we can
use to replicate the behavior of ##__VA_ARGS__ when we eventually enable it.

[1] https://en.cppreference.com/w/cpp/preprocessor/replace
[2] https://github.com/gcc-mirror/gcc/blob/6d6279f88e9678c458bc22ea3be03e73f462f9e7/libcpp/macro.c#L1167

R=thakis

Change-Id: I6c0f3e64ad21b6270822011733d44098537b8f02
Reviewed-on: https://chromium-review.googlesource.com/1181657Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Sergiy Byelozyorov <sergiyb@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584562}