cmake build: Make it easier to build against system litehtml

By first checking for it in the system, then if the internal copy
is checked out.

Fixes: QTCREATORBUG-25144
Change-Id: I839591cc8e311948a21685384de574e7330a6ee5
Reviewed-by: Cristian Adam <cristian.adam@qt.io>

src/plugins/help/CMakeLists.txt

@@ -60,14 +60,8 @@ extend_qtc_plugin(Help
     webenginehelpviewer.h
 )
 
-if (EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/qlitehtml/litehtml/CMakeLists.txt)
+find_package(litehtml QUIET)
-  add_subdirectory(qlitehtml)
+add_subdirectory(qlitehtml)
-else()
-  find_package(litehtml QUIET)
-  if (TARGET litehtml)
-    add_subdirectory(qlitehtml)
-  endif()
-endif()
 
 extend_qtc_plugin(Help
   CONDITION TARGET litehtml AND TARGET qlitehtml

src/plugins/help/qlitehtml/CMakeLists.txt

@@ -1,4 +1,5 @@
-if (EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/litehtml/CMakeLists.txt)
+find_package(litehtml QUIET)
+if(NOT TARGET litehtml AND EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/litehtml/CMakeLists.txt)
     set(ORIG_FPIC ${CMAKE_POSITION_INDEPENDENT_CODE})
     if (WIN32)
       set(LITEHTML_UTF8 ON CACHE BOOL "")
@@ -16,11 +17,10 @@ if (EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/litehtml/CMakeLists.txt)
         target_compile_options(litehtml PRIVATE -O2)
       endif()
     endif()
-else()
-    find_package(litehtml REQUIRED)
 endif()
 
 add_qtc_library(qlitehtml
+  CONDITION TARGET litehtml
   PUBLIC_INCLUDES ${CMAKE_CURRENT_SOURCE_DIR}
   DEPENDS Qt5::Widgets litehtml
   PROPERTIES