Add thread safety configuration check.

include/utils/SkCondVar.h

Index: include/utils/SkCondVar.h
diff --git a/include/utils/SkCondVar.h b/include/utils/SkCondVar.h
index 861a2aba8a057341d26f65110d8711daf7c50585..6f18e1a651df9e0443d00f1a6f3ba9ae9461e5c9 100644
--- a/include/utils/SkCondVar.h
+++ b/include/utils/SkCondVar.h
@@ -8,10 +8,20 @@
 #ifndef SkCondVar_DEFINED
 #define SkCondVar_DEFINED
 
+/**
+ * Import any thread model setting from configuration files.

[reed1, 2014/07/08 13:19:55]

Does this comment belong here, or above <pthread.h> ?

[scroggo, 2014/07/08 13:23:44]

The include of SkTypes.h is to make sure that SK_USE_POSIX_THREADS or
SK_BUILD_FOR_WIN32 is appropriately defined. I think this is the right place for
a comment (although perhaps the comment is not clear enough?).

+ */
+#include "SkTypes.h"
+
 #ifdef SK_USE_POSIX_THREADS
 #include <pthread.h>
 #elif defined(SK_BUILD_FOR_WIN32)
 #include <windows.h>
+#else
+/**
+ * Warn if the implementation of this class is empty, i.e. thread safety is not working.
+ */
+#warning "Thread safety class SkCondVar has no implementation!"
 #endif
 
 /**