configure: Check for -Werror causing failures when compiling tests

Add support for checking whether test case code can compile without
warnings, by recompiling each successful test with -Werror. If the
-Werror version doesn't pass, we bail out. This gives us the same
level of visibility of warnings in test code as --enable-werror
provides for the main compile.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>