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/cpptools/CMakeLists.txt

@@ -112,8 +112,9 @@ add_qtc_plugin(CppTools
     wrappablelineedit.cpp wrappablelineedit.h
 )
 
-if (WITH_TESTS)
-  target_sources(CppTools PRIVATE
+extend_qtc_plugin(CppTools
+  CONDITION WITH_TESTS
+  SOURCES
     cppcodegen_test.cpp
     cppcompletion_test.cpp
     cppheadersource_test.cpp
@@ -127,5 +128,4 @@ if (WITH_TESTS)
     modelmanagertesthelper.cpp modelmanagertesthelper.h
     symbolsearcher_test.cpp
     typehierarchybuilder_test.cpp
-  )
-endif()
+)