Split warning flags list for C and C++

Split warning flags list for C and C++

When using Ccache [1], some flags specific to C are accepted by the C++
compiler but result in warning messages on each invocation of the
compiler. To remediate this, split the warning flags detection list in
three, a common base and a specific list for C and C++.

[1] https://github.com/ccache/ccache/issues/738

Change-Id: I9ef360efbfae445845ca1016e5f5eebdd3bdb0ac
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>