Fix SOURCE_DIR in HandleGTest.cmake (#705)

If benchmark added as cmake subproject, HandleGTest throws an error as  does return absolute source dir.
Change it to , so it will be refering to it's own source dir.

Also see PR #703.

cmake/HandleGTest.cmake

@@ -76,11 +76,11 @@ macro(build_external_gtest)
 endmacro(build_external_gtest)
 
 if (BENCHMARK_ENABLE_GTEST_TESTS)
-  if (IS_DIRECTORY ${CMAKE_SOURCE_DIR}/googletest)
-    set(GTEST_ROOT "${CMAKE_SOURCE_DIR}/googletest")
+  if (IS_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/googletest)
+    set(GTEST_ROOT "${CMAKE_CURRENT_SOURCE_DIR}/googletest")
     set(INSTALL_GTEST OFF CACHE INTERNAL "")
     set(INSTALL_GMOCK OFF CACHE INTERNAL "")
-    add_subdirectory(${CMAKE_SOURCE_DIR}/googletest)
+    add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/googletest)
     set(GTEST_BOTH_LIBRARIES gtest gmock gmock_main)
     foreach(HEADER test mock)
       # CMake 2.8 and older don't respect INTERFACE_INCLUDE_DIRECTORIES, so we