make BUILD_WITH_QT4 off by default

Rationale:
When Qt5 is not found, the script will automatically use Qt4.
However, without this change, when Qt4 is not found, it will not
try to find Qt5 because BUILD_WITH_QT4 is ON by default forbidding the
use of Qt5 even if it is installed.

Also, other packages default to Qt5 including qtkeychain

See also issue #1418

cmake/modules/QtVersionAbstraction.cmake

@@ -2,7 +2,7 @@ include (MacroOptionalFindPackage)
 include (MacroLogFeature)
 
 
-option(BUILD_WITH_QT4 "Build with Qt4 no matter if Qt5 was found" ON)
+option(BUILD_WITH_QT4 "Build with Qt4 no matter if Qt5 was found" OFF)
 
 if( NOT BUILD_WITH_QT4 )
     find_package(Qt5Core QUIET)