Prefix googletest binaries under its own subtree instead of `gtest`

Building all test binaries under their respective subtrees makes
building the project via cmake easier to grok without additional hacks.
In particular, when dealing with the conversion I proposed in
https://reviews.freebsd.org/D19430 (switching from autotools to cmake),
I ran into unexpected gtest prefixing under the googlemock directory, as
opposed to the googletest directory. Example:

Before: `googlemock/gtest/googletest-break-on-failure-unittest_`
After:  `googletest/googletest-break-on-failure-unittest_`

The latter form is easier to translate to packaging manifests when
building googlemock is disabled, as well as enabled, as the path remains
consistent when the feature flag is disabled.

Signed-off-by: Enji Cooper <yaneurabeya@gmail.com>

googlemock/CMakeLists.txt

@@ -49,7 +49,7 @@ endif()
 # targets to the current scope.  We are placing Google Test's binary
 # directory in a subdirectory of our own as VC compilation may break
 # if they are the same (the default).
-add_subdirectory("${gtest_dir}" "${gmock_BINARY_DIR}/gtest")
+add_subdirectory("${gtest_dir}" "${gmock_BINARY_DIR}/${gtest_dir}")
 
 
 # These commands only run if this is the main project