CMake build system: Allow configuration without main app

Quite handy when working only with tetst.

Change-Id: I018e703c3d4d47618b99ef183d930f8d015eeed8
Reviewed-by: Cristian Adam <cristian.adam@qt.io>

src/app/CMakeLists.txt

@@ -19,6 +19,10 @@ add_qtc_executable(qtcreator
   DESTINATION "${IDE_APP_PATH}"
 )
 
+if (NOT TARGET qtcreator)
+  return()
+endif()
+
 if (WIN32)
   set(RC_APPLICATION_NAME "${IDE_DISPLAY_NAME}")
   set(RC_VERSION "${IDE_VERSION}.0")