scons: Add werror explicitly when checking for compatible options.
scons: Add werror explicitly when checking for compatible options.

When checking whether a given option is supported, we try to compile or
link a small test program and see if that succeeded or not. There are
some options which are not really supported, but if you use them the
compiler might produce an error instead of failing to build. An example
of that is clang (version 9 at least) which will warn if you try to
disabled a warning it doesn't know about.

On the develop branch, this is fine since we have werror enabled, and
even these warnings will prevent the options from being enabled. On the
release branches where we disable werror for better future
compatibility, this causes a problem because these options still pass
the test and end up being enabled. This doesn't break the build, but
can cause a bunch of annoying and unnecessary warnings which clutter
up the build output.

To remove this inconsistency, we can enable werror just for the test
compiles or links which decide whether or not an option is supported.
That won't necessarily differentiate *why* a warning was generated, so
if through some strange combination of circumstances something else
causes a warning pervasively through the build, then this may think
an option is to blame and not enable it even though it's fine. This
seems unlikely though, and not worth worrying about without a specific
example showing that it can/will actually happen.

Change-Id: I6a1e3273c0b646a5636dc9986c70dcd5332f6f64
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/54624
Maintainer: Gabe Black <5c8aa1266c03186393d9e7b3b4e3b14239af8201@google.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Bobby Bruce <b9dba908ffb6c6d8ffe449cc88b3709c13c485c2@ucdavis.edu>

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