[LyX/master] Cmake windows build: Fix a cache variable

[Kornel Benko]

commit f4254e69995f54877f642b442c6527a8b4f922f8
Author: Kornel Benko <kor...@lyx.org>
Date:   Mon Jun 24 10:36:37 2019 +0200

    Cmake windows build: Fix a cache variable
    
    Thanks to Jürgen Womser-Schütz
---
 development/cmake/ConfigureChecks.cmake |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/development/cmake/ConfigureChecks.cmake 
b/development/cmake/ConfigureChecks.cmake
index 1b6bf45..5dc0bc5 100644
--- a/development/cmake/ConfigureChecks.cmake
+++ b/development/cmake/ConfigureChecks.cmake
@@ -275,7 +275,7 @@ if(LYX_USE_QT MATCHES "QT5")
     set(lyx_qt5_config "QtCore/qconfig.h")
   endif()
   if(WIN32)
-    set(QT_USES_X11 CACHE "Win32 compiled without X11" 0)
+    set(QT_USES_X11 OFF BOOL CACHE "Win32 compiled without X11")
     # The try_run for minngw would not work here anyway
   else()
     check_cxx_source_runs(