Clone the github by git and allow users to select gtest manually

There are reports saying clonning gtest all the time may failure in case
of bad network condition. So we tried to improve this by clonning with
git protocal and allow users to specify the gtest manually. See the
updated documents for details.

README.md

@@ -83,7 +83,9 @@ If you meet any problem about MSVC Compiler Error C4737. Try to add the /EHa opt
 # Develop async_simple
 
 The build of async_simple requires libaio, googletest and cmake.  Both libaio and googletest
-are optional. (Testing before using is highly suggested.)
+are optional. (Testing before using is highly suggested.) By default, async_simple would try
+to clone the googletest from git to make sure the version used is correct. But in case the
+users have problems with networks, users can try to install the gtest libraries by the following instructions and use the CMake variables ('GMOCK_INCLUDE_DIR', 'GTEST_LIBRARIES', 'GMOCK_INCLUDE_DIR', 'GMOCK_LIBRARIES') to specify the location.
 
 ## Using apt (Ubuntu and Debian)
 
@@ -186,7 +188,7 @@ $ mkdir build && cd build
 # Specify [-DASYNC_SIMPLE_ENABLE_TESTS=OFF] to skip tests.
 # Specify [-DASYNC_SIMPLE_BUILD_DEMO_EXAMPLE=OFF] to skip build demo example.
 # Specify [-DASYNC_SIMPLE_DISABLE_AIO=ON] to skip the build libaio
-CXX=clang++ CC=clang cmake ../ -DCMAKE_BUILD_TYPE=[Release|Debug] [-DASYNC_SIMPLE_ENABLE_TESTS=OFF] [-DASYNC_SIMPLE_BUILD_DEMO_EXAMPLE=OFF] [-DASYNC_SIMPLE_DISABLE_AIO=ON]
+CXX=clang++ CC=clang cmake ../ -DCMAKE_BUILD_TYPE=[Release|Debug] [-DASYNC_SIMPLE_ENABLE_TESTS=OFF] [-DASYNC_SIMPLE_BUILD_DEMO_EXAMPLE=OFF] [-DASYNC_SIMPLE_DISABLE_AIO=ON] [-DGMOCK_INCLUDE_DIR=<path-to-headers of gtest> -DGTEST_INCLUDE_DIR=<path-to-headers of mock> -DGTEST_LIBRARIES=<path-to-library-of-gtest>  -DGMOCK_LIBRARIES=<path-to-library-of-gmock> ]
 # for gcc, use CXX=g++ CC=gcc
 make -j4
 make test # optional

README_CN.md

@@ -79,6 +79,9 @@ async\_simple 涉及 C++20 协程，对编译器版本有较高要求。需要 c
 
 本项目编译运行测试需要 gtest和gmock，需要 libaio 的示例代码则依赖 libaio。
 
+默认情况下，async_simple 会尝试从 git 拉取 gtest 源码以保证版本一致性。在用户存在网络的情况下，
+建议用户按照以下指令安装对应版本的 gtest 并使用 CMake 变量 ('GMOCK_INCLUDE_DIR', 'GTEST_LIBRARIES', 'GMOCK_INCLUDE_DIR', 'GMOCK_LIBRARIES') 指定安装的 gtest 所在的位置。
+
 ## Debian/Ubuntu系列
 
 - 安装clang11及其以上版本。安装方法见：[APT Packages](https://apt.llvm.org/)
@@ -181,7 +184,7 @@ mkdir build && cd build
 # Specify [-DASYNC_SIMPLE_ENABLE_TESTS=OFF] to skip tests.
 # Specify [-DASYNC_SIMPLE_BUILD_DEMO_EXAMPLE=OFF] to skip build demo example.
 # Specify [-DASYNC_SIMPLE_DISABLE_AIO=ON] to skip the build libaio
-CXX=clang++ CC=clang cmake ../ -DCMAKE_BUILD_TYPE=[Release|Debug] [-DASYNC_SIMPLE_ENABLE_TESTS=OFF] [-DASYNC_SIMPLE_BUILD_DEMO_EXAMPLE=OFF] [-DASYNC_SIMPLE_DISABLE_AIO=ON]
+CXX=clang++ CC=clang cmake ../ -DCMAKE_BUILD_TYPE=[Release|Debug] [-DASYNC_SIMPLE_ENABLE_TESTS=OFF] [-DASYNC_SIMPLE_BUILD_DEMO_EXAMPLE=OFF] [-DASYNC_SIMPLE_DISABLE_AIO=ON] [-DGMOCK_INCLUDE_DIR=<path-to-headers of gtest> -DGTEST_INCLUDE_DIR=<path-to-headers of mock> -DGTEST_LIBRARIES=<path-to-library-of-gtest>  -DGMOCK_LIBRARIES=<path-to-library-of-gmock> ]
 make -j4
 make test
 make install

cmake/FindGTest.cmake

@@ -1,18 +1,26 @@
-message(STATUS "fetch GTest from https://github.com/google/googletest.git")
-include(FetchContent)
-FetchContent_Declare(
-    googletest
-    GIT_REPOSITORY https://github.com/google/googletest.git
-    GIT_TAG release-1.11.0
-)
-set(gtest_force_shared_crt ON CACHE BOOL "" FORCE)
-set(BUILD_GMOCK ON CACHE BOOL "" FORCE)
-set(BUILD_GTEST ON CACHE BOOL "" FORCE)
-FetchContent_MakeAvailable(googletest)
-set(GMOCK_INCLUDE_DIR ${CMAKE_BINARY_DIR}/_deps/googletest-src/googlemock/include/)
-set(GTEST_INCLUDE_DIR ${CMAKE_BINARY_DIR}/_deps/googletest-src/googletest/include/)
-set(GTEST_LIBRARIES gtest_main)
-set(GMOCK_LIBRARIES gmock_main)
+if (DEFINED GMOCK_INCLUDE_DIR AND DEFINED GTEST_INCLUDE_DIR
+    AND DEFINED GTEST_LIBRARIES AND DEFINED GMOCK_LIBRARIES)
+    message(STATUS "Using GTest from specified path")
+else()
+    message(STATUS "Not all GTest variable got specified: "
+                   "'GMOCK_INCLUDE_DIR', 'GTEST_INCLUDE_DIR', "
+                   "'GTEST_LIBRARIES', 'GMOCK_LIBRARIES'")
+    message(STATUS "fetch GTest from git@github.com:google/googletest.git")
+    include(FetchContent)
+    FetchContent_Declare(
+        googletest
+        GIT_REPOSITORY git@github.com:google/googletest.git
+        GIT_TAG release-1.11.0
+    )
+    set(gtest_force_shared_crt ON CACHE BOOL "" FORCE)
+    set(BUILD_GMOCK ON CACHE BOOL "" FORCE)
+    set(BUILD_GTEST ON CACHE BOOL "" FORCE)
+    FetchContent_MakeAvailable(googletest)
+    set(GMOCK_INCLUDE_DIR ${CMAKE_BINARY_DIR}/_deps/googletest-src/googlemock/include/)
+    set(GTEST_INCLUDE_DIR ${CMAKE_BINARY_DIR}/_deps/googletest-src/googletest/include/)
+    set(GTEST_LIBRARIES gtest_main)
+    set(GMOCK_LIBRARIES gmock_main)
+endif()
 
 message(STATUS "GTest: ${GTEST_INCLUDE_DIR}, ${GTEST_LIBRARIES}")
 message(STATUS "GMock: ${GMOCK_INCLUDE_DIR}, ${GMOCK_LIBRARIES}")