Fix SOURCE_DIR in HandleGTest.cmake (google#703)

* Fix SOURCE_DIR in HandleGTest.cmake

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.

cmake/HandleGTest.cmake

@@ -75,27 +75,34 @@ macro(build_external_gtest)
   endforeach()
 endmacro(build_external_gtest)
 
+macro(build_gtest_from_dir)
+  set(INSTALL_GTEST OFF CACHE INTERNAL "")
+  set(INSTALL_GMOCK OFF CACHE INTERNAL "")
+  add_subdirectory(${GTEST_ROOT} build)
+  set(GTEST_BOTH_LIBRARIES gtest gmock gmock_main)
+  foreach(HEADER test mock)
+    # CMake 2.8 and older don't respect INTERFACE_INCLUDE_DIRECTORIES, so we
+    # have to add the paths ourselves.
+    set(HFILE g${HEADER}/g${HEADER}.h)
+    set(HPATH ${GTEST_ROOT}/google${HEADER}/include)
+    find_path(HEADER_PATH_${HEADER} ${HFILE}
+            NO_DEFAULT_PATHS
+            HINTS ${HPATH}
+            )
+    if (NOT HEADER_PATH_${HEADER})
+      message(FATAL_ERROR "Failed to find header ${HFILE} in ${HPATH}")
+    endif()
+    list(APPEND GTEST_INCLUDE_DIRS ${HEADER_PATH_${HEADER}})
+  endforeach()
+endmacro()
+
 if (BENCHMARK_ENABLE_GTEST_TESTS)
   if (IS_DIRECTORY ${CMAKE_SOURCE_DIR}/googletest)
     set(GTEST_ROOT "${CMAKE_SOURCE_DIR}/googletest")
-    set(INSTALL_GTEST OFF CACHE INTERNAL "")
-    set(INSTALL_GMOCK OFF CACHE INTERNAL "")
-    add_subdirectory(${CMAKE_SOURCE_DIR}/googletest)
-    set(GTEST_BOTH_LIBRARIES gtest gmock gmock_main)
-    foreach(HEADER test mock)
-      # CMake 2.8 and older don't respect INTERFACE_INCLUDE_DIRECTORIES, so we
-      # have to add the paths ourselves.
-      set(HFILE g${HEADER}/g${HEADER}.h)
-      set(HPATH ${GTEST_ROOT}/google${HEADER}/include)
-      find_path(HEADER_PATH_${HEADER} ${HFILE}
-          NO_DEFAULT_PATHS
-          HINTS ${HPATH}
-      )
-      if (NOT HEADER_PATH_${HEADER})
-        message(FATAL_ERROR "Failed to find header ${HFILE} in ${HPATH}")
-      endif()
-      list(APPEND GTEST_INCLUDE_DIRS ${HEADER_PATH_${HEADER}})
-    endforeach()
+    build_gtest_from_dir()
+  elseif (IS_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/googletest)
+    set(GTEST_ROOT "${CMAKE_CURRENT_SOURCE_DIR}/googletest")
+    build_gtest_from_dir()
   elseif(BENCHMARK_DOWNLOAD_DEPENDENCIES)
     build_external_gtest()
   else()