configure: Use -std=gnu11

Now that the minimum gcc version is 7.5, we can use C11.
This will allow lots of cleanups to the code, currently
hidden behind macros in include/qemu/compiler.h.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20210614233143.1221879-2-richard.henderson@linaro.org>