Make CMakeLists.txt current to CMake 3.8 features

This commit updates the CMake files to use newer CMake features.

GSL is now a INTERFACE and one only need to link against that interface
(include paths and natvis files will be taken care automatically).

Whene generating a VS project test will be properly nested into a test
subfolder.

if(compiler) expression were modified to generator expressions.

CMakeLists.txt

@@ -1,10 +1,20 @@
-cmake_minimum_required(VERSION 2.8.7)
+cmake_minimum_required(VERSION 3.8.0)
 
 project(GSL CXX)
 
-install(
-    DIRECTORY include/gsl
-    DESTINATION include
+set_property(GLOBAL PROPERTY USE_FOLDERS ON)
+
+add_library(cppGSL INTERFACE)
+set(CMAKE_CXX_EXTENSIONS OFF)
+target_compile_features(cppGSL INTERFACE cxx_std_14)
+
+target_include_directories(cppGSL INTERFACE
+    $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
+    $<INSTALL_INTERFACE:include/gsl>
+)
+
+target_sources(cppGSL INTERFACE 
+    ${CMAKE_CURRENT_SOURCE_DIR}/GSL.natvis
 )
 
 enable_testing()

tests/CMakeLists.txt

@@ -1,36 +1,47 @@
-cmake_minimum_required(VERSION 2.8.7)
+cmake_minimum_required(VERSION 3.8.0)
 
 project(GSLTests CXX)
 
 if (NOT EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/unittest-cpp/tests)
-    execute_process(COMMAND git submodule update --init WORKING_DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}")
+    find_package(Git)
+    execute_process(COMMAND ${GIT_EXECUTABLE} submodule update --init
+                    WORKING_DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}")
 endif()
 
 add_subdirectory(unittest-cpp)
 
-include_directories(
-    ../include
-    ./unittest-cpp
+add_library(unittest_cpp INTERFACE)
+target_include_directories(unittest_cpp INTERFACE
+    $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/unittest-cpp>
 )
 
-add_definitions(-DGSL_THROW_ON_CONTRACT_VIOLATION)
-
-if(MSVC) # has the support we need
-    # remove unnecessary warnings about unchecked iterators
-    add_definitions(-D_SCL_SECURE_NO_WARNINGS)
-    add_compile_options(/EHsc /W4 /WX)
-else()
-    include(CheckCXXCompilerFlag)
-    CHECK_CXX_COMPILER_FLAG("-std=c++14" COMPILER_SUPPORTS_CXX14)
-    CHECK_CXX_COMPILER_FLAG("-std=c++11" COMPILER_SUPPORTS_CXX11)
-    if(COMPILER_SUPPORTS_CXX14)
-        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fno-strict-aliasing -std=c++14 -Werror -Wall -Wextra -Wpedantic -Wno-missing-braces -Wconversion -Wsign-conversion -Wctor-dtor-privacy -Wshadow -Wnon-virtual-dtor -Wold-style-cast -Wcast-align -Woverloaded-virtual")
-    elseif(COMPILER_SUPPORTS_CXX11)
-        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fno-strict-aliasing -std=c++11 -Werror -Wall -Wextra -Wpedantic -Wno-missing-braces -Wconversion -Wsign-conversion -Wctor-dtor-privacy -Wshadow -Wnon-virtual-dtor -Wold-style-cast -Wcast-align -Woverloaded-virtual")
-    else()
-      message(STATUS "The compiler ${CMAKE_CXX_COMPILER} has no C++11 support. Please use a different C++ compiler.")
-    endif()
-endif()
+add_library(gsl_tests_config INTERFACE)
+target_compile_definitions(gsl_tests_config INTERFACE
+    GSL_THROW_ON_CONTRACT_VIOLATION
+    $<$<CXX_COMPILER_ID:MSVC>:_SCL_SECURE_NO_WARNINGS>
+)
+target_compile_options(gsl_tests_config INTERFACE
+    $<$<CXX_COMPILER_ID:MSVC>:
+        /EHsc
+        /W4
+    >
+    $<$<NOT:$<CXX_COMPILER_ID:MSVC>>:
+        -fno-strict-aliasing
+        -Wall
+        -Wcast-align
+        -Wconversion
+        -Wctor-dtor-privacy
+        -Werror
+        -Wextra
+        -Wno-missing-braces
+        -Wnon-virtual-dtor
+        -Wold-style-cast
+        -Woverloaded-virtual
+        -Wpedantic
+        -Wshadow
+        -Wsign-conversion
+    >
+)
 
 function(add_gsl_test name)
     add_executable(${name} ${name}.cpp)
@@ -39,6 +50,8 @@ function(add_gsl_test name)
       ${name}
       ${name}
     )
+    set_property(TARGET ${name} PROPERTY FOLDER "gsl_tests")
+    target_link_libraries(${name} cppGSL gsl_tests_config unittest_cpp)
 endfunction()
 
 add_gsl_test(span_tests)