do not detect or configure iconv for Windows

Qt doesn't use iconv on Windows, but configuring it will appear to
work and the build will complete. The result is that character
set conversions do not work.

Configure.exe already disables iconv for Windows.

backported from qt5/qtbase
Change-Id: I449a00860c2e77e6cdd8cdcf7108621c684207bf
Reviewed-by: Lars Knoll <[email protected]>
Reviewed-by: Friedemann Kleint <[email protected]>
(cherry picked from commit 9fa2b641ba6ff4f4b3f474b87ddb642cfa5c3d83)

Author: mabrand

configure

@@ -5831,7 +5831,7 @@ fi
 
 # auto-detect iconv(3) support
 if [ "$CFG_ICONV" != "no" ]; then
-    if [ "$PLATFORM_QWS" = "yes" ] || [ "$PLATFORM_QPA" = "yes" -a "$CFG_ICONV" = "auto" ]; then
+    if [ "$PLATFORM_QWS" = "yes" -o "$XPLATFORM_MINGW" = "yes" ] || [ "$PLATFORM_QPA" = "yes" -a "$CFG_ICONV" = "auto" ]; then
         CFG_ICONV=no
     elif compileTest "unix/iconv" "POSIX iconv"; then
         CFG_ICONV=yes