CMake build: Add ability to disable building of individual plugins

Adds a cache entry "BUILD_PLUGIN_${NAME}", defaulting to ON which
can be set to OFF to disable building of a plugin.

Adds a extend_qtc_plugin function that should be used to add
properties to a plugin after add_qtc_plugin, instead of the
standard CMake functions target_... . The new function results
in a no-op if the plugin was disabled.

Change-Id: I57f6799620aea0aaa8b56acead4815ccced95911
Reviewed-by: Cristian Adam <cristian.adam@qt.io>
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>

src/plugins/clangcodemodel/CMakeLists.txt

@@ -47,11 +47,11 @@ add_qtc_plugin(ClangCodeModel
   EXPLICIT_MOC clangcodemodelplugin.h
 )
 
-if (WITH_TESTS)
-  target_sources(ClangCodeModel PRIVATE
+extend_qtc_plugin(ClangCodeModel
+  CONDITION WITH_TESTS
+  SOURCES
     test/clangautomationutils.cpp test/clangautomationutils.h
     test/clangbatchfileprocessor.cpp test/clangbatchfileprocessor.h
     test/clangcodecompletion_test.cpp test/clangcodecompletion_test.h
     test/data/clangtestdata.qrc
- )
-endif()
+)