cmake: Add fallback in case QTQCHDIR was not found for KF5 build variant

This is required at least for the openSUSE distribution Leap 15.1, which
uses ECM version 5.55 by default.

CCBUG:427070

CMakeLists.txt

@@ -81,6 +81,9 @@ else()
     set(KDEV_MIN_VERSION 5.1.2)
     set(KDEV_SOURCE_DIR lib/kdev5-php)
     set(KDEV_COMPONENTS KDev::Interfaces KDev::Language KDev::Shell)
+    if(NOT QTQCHDIR)
+        set(QTQCHDIR ${KDE_INSTALL_QTQCHDIR})
+    endif()
 endif()
 
 # search packages used by Qt