Different versions of GCC and Clang use different versions of the C standard.
This repeatedly caused problems already, e.g. with duplicated typedefs:

 https://lists.gnu.org/archive/html/qemu-devel/2018-11/msg05829.html

or with for-loop variable initializers:

 https://lists.gnu.org/archive/html/qemu-devel/2019-01/msg00237.html

To avoid these problems, we should enforce the C language version to the
same level for all compilers. Since our minimum compiler versions is
GCC v4.8, our best option is "gnu99" for C code right now ("gnu17" is not
available there yet, and "gnu11" is marked as "experimental"), and "gnu++98"
for the few C++ code that we have in the repository.
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: NRichard Henderson <richard.henderson@linaro.org>
Reviewed-by: NGreg Kurz <groug@kaod.org>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>
Reviewed-by: NAlex Bennée <alex.bennee@linaro.org>
Signed-off-by: NThomas Huth <thuth@redhat.com>