cmake: Use a standard option for controlling the tests

As per the docs [1], there's a standard variable for this purpose. This
introduces a behavior change, the tests are now being built by default.

[1] https://cmake.org/cmake/help/v3.0/module/CTest.html

CMakeLists.txt

@@ -5,6 +5,7 @@
 
 cmake_minimum_required(VERSION 3.1)
 project(spdlog VERSION 1.0.0)
+include(CTest)
 
 set(CMAKE_CXX_STANDARD 11)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
@@ -12,7 +13,6 @@ set(CMAKE_CXX_STANDARD_REQUIRED ON)
 add_library(spdlog INTERFACE)
 
 option(SPDLOG_BUILD_EXAMPLES "Build examples" OFF)
-option(SPDLOG_BUILD_TESTS "Build tests" OFF)
 
 target_include_directories(
     spdlog
@@ -28,7 +28,7 @@ if(SPDLOG_BUILD_EXAMPLES)
     add_subdirectory(example)
 endif()
 
-if(SPDLOG_BUILD_TESTS)
+if(BUILD_TESTING)
     add_subdirectory(tests)
 endif()