configure: mark qemu-ga VSS includes as system headers

configure: mark qemu-ga VSS includes as system headers

As of e4650c81, we do w32 builds with -Werror enabled. Unfortunately
for cases where we enable VSS support in qemu-ga, we still have
warnings generated by VSS includes that ship as part of the Microsoft
VSS SDK.

We can selectively address a number of these warnings using

  #pragma GCC diagnostic ignored ...

but at least one of these:

  warning: ‘typedef’ was ignored in this declaration

resulting from declarations of the form:

  typedef struct Blah { ... };

does not provide a specific command-line/pragma option to disable
warnings of the sort.

To allow VSS builds to succeed, the next-best option is disabling
these warnings on a per-file basis. pragmas like #pragma GCC
system_header can be used to declare subsequent includes/declarations
as being exempt from normal warnings, but this must be done within
a header file.

Since we don't control the VSS SDK, we'd need to rely on a
intermediate header include to accomplish this, and
since different objects in the VSS link target rely on different
headers from the VSS SDK, this would become somewhat of a rat's nest
(though not totally unmanageable).

The next step up in granularity is just marking the entire VSS
SDK include path as system headers via -isystem. This is a bit more
heavy-handed, but since this SDK hasn't changed since 2005, there's
likely little to be gained from selectively disabling warnings
anyway, so we implement that approach here.

This fixes the -Werror failures in both the configure test and the
qga build due to shared reliance on $vss_win32_include. For the
same reason, this also enforces a new dependency on -isystem support
in the C/C++ compiler when building QGA with VSS enabled.

Cc: Thomas Huth <thuth@redhat.com>
Cc: Stefan Weil <sw@weilnetz.de>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>