Project project_BUILD_* variables (#23)

Author: woodfell

SwiftCmakeOptions.cmake

@@ -164,6 +164,15 @@ function(swift_create_project_options)
     set(x_PROJECT ${PROJECT_NAME})
   endif()
 
+  if(NOT ${x_PROJECT}_BUILD_VARS_PROTECTED)
+    foreach(feat "TESTS" "TEST_LIBS" "EXAMPLES" "DOCS")
+      if(DEFINED ${x_PROJECT}_BUILD_${feat})
+        message(FATAL_ERROR "Something or someone has set ${x_PROJECT}_BUILD_${feat}. This is an internal option and must not be set from anywhere. Use ${x_PROJECT}_ENABLE_${feat} instead.")
+      endif()
+    endforeach()
+    set(${x_PROJECT}_BUILD_VARS_PROTECTED ON CACHE BOOL "Checked that nothing has set any BUILD vars manually")
+  endif()
+
   set(tests_possible ON)
   set(test_libs_possible ON)
   if(x_DISABLE_TEST_COMPONENTS)