configure.ac: append to existing MSVC compat flags

Currently we were overwriting the existing warning flags, instead of
adding new [as applicable].

v2: Add missing space before -Werror (Eric)

Fixes e4b2b69e828 ("configure: Add and use AX_CHECK_COMPILE_FLAG")
Cc: Matt Turner <mattst88@gmail.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Matt Turner <mattst88@gmail.com> (v1)
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
1 file changed