Change back to "protobuf_BUILD_TESTS" flag

In anticipation of PR #753 getting merged we'll change this back. However until that PR gets merged the old value "BUILD_TESTS" is really needed.

cmake/README.md

@@ -131,7 +131,7 @@ To create *Visual Studio* solution file:
 It will generate *Visual Studio* solution file *protobuf.sln* in current directory.
 
 If the *gmock* directory does not exist, and you do not want to build protobuf unit tests,
-you need to add *cmake* command argument `-DBUILD_TESTING=OFF` to disable testing.
+you need to add *cmake* command argument `-Dprotobuf_BUILD_TESTS=OFF` to disable testing.
 
 Compiling
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