CMake: include(CTest) only when building tests.

This is needed in order to support usage of this library as a
subdirectory in a parent project.  In that situation, prior
to this change, the inclusion of CTest would unconditionally
enable BUILD_TESTING which would then bleed into other parts
of the project.

Also added some comments explaining how this logic works.

CMakeLists.txt

@@ -5,7 +5,6 @@
 
 cmake_minimum_required(VERSION 3.1)
 project(spdlog VERSION 1.3.0 LANGUAGES CXX)
-include(CTest)
 include(CMakeDependentOption)
 include(GNUInstallDirs)
 
@@ -54,10 +53,22 @@ endif()
 option(SPDLOG_BUILD_EXAMPLES "Build examples" ${SPDLOG_MASTER_PROJECT})
 option(SPDLOG_BUILD_BENCH "Build benchmarks" ${SPDLOG_MASTER_PROJECT})
 
+# Logic for enabling tests: If the user does not explicitly
+# specify the value of the SPDLOG_BUILD_TESTING variable then the
+# logic is simpler to reason about: that is, testing will be
+# built if and only if BUILD_TESTING is ON and we are a Master
+# Project. On the other hand, if the user overrides the value of
+# SPDLOG_BUILD_TESTING then it can get a bit more tricky due to
+# caching.
+
 cmake_dependent_option(SPDLOG_BUILD_TESTING
     "Build spdlog tests" ${SPDLOG_MASTER_PROJECT}
     "BUILD_TESTING" OFF
 )
+if(SPDLOG_BUILD_TESTING)
+    # Include CTest conditionally since it will enable BUILD_TESTING.
+    include(CTest)
+endif()
 
 target_include_directories(
     spdlog