[PATCH] configure: Expand test which disable -Wmissing-braces

[Anthony PERARD via <qemu-devel@nongnu.org>]

From: Anthony PERARD <anthony.perard@citrix.com>

With "clang 6.0.0-1ubuntu2" on Ubuntu Bionic, the test with build
fine, but clang still suggest braces around the zero initializer in a
few places, where there is a subobject. Expand test to include a sub
struct which doesn't build on clang 6.0.0-1ubuntu2, and give:
    config-temp/qemu-conf.c:7:8: error: suggest braces around initialization of subobject [-Werror,-Wmissing-braces]
    } x = {0};
           ^
           {}

These are the error reported by clang on QEMU's code (v7.2.0):
hw/pci-bridge/cxl_downstream.c:101:51: error: suggest braces around initialization of subobject [-Werror,-Wmissing-braces]
    dvsec = (uint8_t *)&(CXLDVSECPortExtensions){ 0 };

hw/pci-bridge/cxl_root_port.c:62:51: error: suggest braces around initialization of subobject [-Werror,-Wmissing-braces]
    dvsec = (uint8_t *)&(CXLDVSECPortExtensions){ 0 };

tests/qtest/virtio-net-test.c:322:34: error: suggest braces around initialization of subobject [-Werror,-Wmissing-braces]
    QOSGraphTestOptions opts = { 0 };

Reported-by: Andrew Cooper <Andrew.Cooper3@citrix.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
---

While Ubuntu Bionic isn't supposed to be supported anymore, clang v6
is still the minimum required as tested by ./configure.
---
 configure | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/configure b/configure
index 9f0bc57546..3cd9b8bad4 100755
--- a/configure
+++ b/configure
@@ -1290,7 +1290,11 @@ fi
 # Disable -Wmissing-braces on older compilers that warn even for
 # the "universal" C zero initializer {0}.
 cat > $TMPC << EOF
+struct s {
+  void *a;
+};
 struct {
+  struct s s;
   int a[2];
 } x = {0};
 EOF
-- 
Anthony PERARD