meson.build: Check for the availability of __attribute__((gcc_struct)) on MSYS2

meson.build: Check for the availability of __attribute__((gcc_struct)) on MSYS2

Since quite a while MSYS2 now supports Clang as a compiler, too.
Unfortunately, this compiler is lacking the __attribute__((gcc_struct))
that we need for compiling on Windows. But since the compiler is
available now, some people started to use it to compile QEMU on MSYS2,
apparently ignoring the compiler warnings (see for example the ticket at
https://gitlab.com/qemu-project/qemu/-/issues/2476 ). These builds are
likely broken in a couple of spots, so let's make sure that we rather
bail out early in the configuration phase instead of allowing the build
to succeed with warnings.

Message-ID: <20240815122719.727639-1-thuth@redhat.com>
Tested-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Thomas Huth <thuth@redhat.com>