From 85b4d7c8d6e45ba32c65b1b75c6e48377c50506c Mon Sep 17 00:00:00 2001
From: "David P. Sicilia" <dpsicilia@gmail.com>
Date: Sat, 1 Dec 2018 20:37:06 -0500
Subject: [PATCH] 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 | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 1dc01f87..7fda48a8 100644
--- a/CMakeLists.txt
+++ b/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