Improve build script for standalone tests (#963)

* Require Git for build tests only if installed GTest is not found

Cloning via Git is not the only path to obtain Google Test framework. In Linux,
using a system package manager is the preferred way, and gtest can be installed
through APK, APT, DNF, pacman, or many other. Now we make Git mandatory after
checking GTest existence.

See also: https://github.com/microsoft/GSL/pull/961#discussion_r548959056

* Support standalone tests

The patch makes possible to run auto-tests against globally installed GSL,
ignoring local headers. To do this, run CMake in the tests/ folder.

    cmake -B build -S tests -DGSL_CXX_STANDARD=14

This feature should not break existing build recipes.

Co-authored-by: Nicholas Guriev <nicholas@guriev.su>

tests/CMakeLists.txt

@@ -1,12 +1,11 @@
 cmake_minimum_required(VERSION 3.0.2)
 
 project(GSLTests CXX)
+enable_testing()  # again, for support standalone testing
 
 include(FindPkgConfig)
 include(ExternalProject)
 
-find_package(Git REQUIRED QUIET)
-
 # will make visual studio generated project group files
 set_property(GLOBAL PROPERTY USE_FOLDERS ON)
 
@@ -16,6 +15,9 @@ endif()
 
 pkg_search_module(GTestMain gtest_main)
 if (NOT GTestMain_FOUND)
+    # No pre-installed GTest is available, try to download it using Git.
+    find_package(Git REQUIRED QUIET)
+
     configure_file(CMakeLists.txt.in googletest-download/CMakeLists.txt)
     execute_process(
         COMMAND ${CMAKE_COMMAND} -G "${CMAKE_GENERATOR}" .
@@ -45,6 +47,13 @@ if (NOT GTestMain_FOUND)
     )
 endif()
 
+if (CMAKE_CURRENT_SOURCE_DIR STREQUAL CMAKE_SOURCE_DIR)
+    # CMake has been started independently in this directory with tests. Do
+    # import the globally installed Guidelines Support Library and test it
+    # instead of the current version from the include/ folder.
+    find_package(Microsoft.GSL REQUIRED)
+endif()
+
 if (MSVC AND (GSL_CXX_STANDARD EQUAL 17))
     set(GSL_CPLUSPLUS_OPT -Zc:__cplusplus -permissive-)
 endif()
@@ -160,7 +169,7 @@ set_property(TARGET  PROPERTY FOLDER "GSL_tests")
 function(add_gsl_test name)
     add_executable(${name} ${name}.cpp)
     target_link_libraries(${name}
-        GSL
+        Microsoft.GSL::GSL
         gsl_tests_config
         ${GTestMain_LIBRARIES}
     )
@@ -262,7 +271,7 @@ endif(MSVC)
 function(add_gsl_test_noexcept name)
     add_executable(${name} ${name}.cpp)
     target_link_libraries(${name}
-        GSL
+        Microsoft.GSL::GSL
         gsl_tests_config_noexcept
         ${GTestMain_LIBRARIES}
     )