Use ExternalProject_Add rather than including the gtest directory

CMakeLists.txt

@@ -56,6 +56,9 @@ IF(WIN32)
     ADD_DEFINITIONS(-DNOMINMAX)
 ENDIF(WIN32)
 
+
+INCLUDE(ExternalProject)
+
 # *******************
 # Compilation options
 # *******************
@@ -160,11 +163,23 @@ ADD_SUBDIRECTORY(src)
 IF(EXISTS "${CMAKE_SOURCE_DIR}/googletest/CMakeLists.txt")
     # Enable tests and add the test directory
     MESSAGE(STATUS "Tests have been enabled")
-    SET(gtest_force_shared_crt ON CACHE BOOL "Always use /MD")
-    SET(BUILD_GMOCK OFF)
-    SET(BUILD_GTEST ON)
+    SET(GOOGLETEST_ROOT ${CMAKE_SOURCE_DIR}/googletest)
+    SET(GOOGLETEST_INCLUDE ${GOOGLETEST_ROOT}/googletest/include)
+    SET(GOOGLETEST_BINARY_DIR ${CMAKE_CURRENT_BINARY_DIR}/googletest)
+    SET(GOOGLETEST_LIBRARY ${GOOGLETEST_BINARY_DIR}/googletest)
+
+    ExternalProject_Add(
+        googletest
+        DOWNLOAD_COMMAND ""
+        SOURCE_DIR ${GOOGLETEST_ROOT}
+        BINARY_DIR ${GOOGLETEST_BINARY_DIR}
+        CMAKE_CACHE_ARGS "-DBUILD_GTEST:bool=ON" "-DBUILD_GMOCK:bool=OFF"
+                         "-Dgtest_force_shared_crt:bool=ON"
+        INSTALL_COMMAND ""
+    )
+    # Make sure we build googletest before anything else
+    ADD_DEPENDENCIES(tins googletest)
     ENABLE_TESTING()
-    ADD_SUBDIRECTORY(googletest)
     ADD_SUBDIRECTORY(tests)
 ELSE()
     MESSAGE(STATUS "googletest git submodule is absent. Run `git submodule init && git submodule update` to get it")