qga/vss-win32: Fix ConvertStringToBSTR redefinition with newer MinGW

Newer versions of MinGW-w64 provide ConvertStringToBSTR() in the
_com_util namespace via <comutil.h>. This causes a redefinition
error when building qemu-ga on Windows with these toolchains.

Add a meson check to detect whether ConvertStringToBSTR is already
available, and conditionally compile our fallback implementation
only when the system does not provide one.

Signed-off-by: Nguyen Dinh Phi <phind.uet@gmail.com>
Suggested-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Reviewed-by: Kostiantyn Kostiuk <kkostiuk@redhat.com>
Message-ID: <20251218085446.462827-2-phind.uet@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>

meson.build

@@ -3241,6 +3241,18 @@ endif
 # Detect host pointer size for the target configuration loop.
 host_long_bits = cc.sizeof('void *') * 8
 
+# Detect if ConvertStringToBSTR has been defined in _com_util namespace
+if host_os == 'windows'
+  has_convert_string_to_bstr = cxx.links('''
+    #include <comutil.h>
+    int main() {
+        BSTR b = _com_util::ConvertStringToBSTR("test");
+        return b ? 0 : 1;
+    }
+  ''')
+  config_host_data.set('CONFIG_CONVERT_STRING_TO_BSTR', has_convert_string_to_bstr)
+endif
+
 ########################
 # Target configuration #
 ########################

qga/vss-win32/install.cpp

@@ -549,6 +549,7 @@ STDAPI DllUnregisterServer(void)
 
 
 /* Support function to convert ASCII string into BSTR (used in _bstr_t) */
+#ifndef CONFIG_CONVERT_STRING_TO_BSTR
 namespace _com_util
 {
     BSTR WINAPI ConvertStringToBSTR(const char *ascii) {
@@ -566,6 +567,7 @@ namespace _com_util
         return bstr;
     }
 }
+#endif
 
 /* Stop QGA VSS provider service using Winsvc API  */
 STDAPI StopService(void)