Enable running the tests against an installed copy of spdlog

tests/CMakeLists.txt

@@ -1,5 +1,12 @@
+cmake_minimum_required(VERSION 3.2)
+
 project(spdlog_utests CXX)
 
+if(NOT TARGET spdlog)
+    # Stand-alone build
+    find_package(spdlog REQUIRED)
+endif()
+
 include(../cmake/utils.cmake)
 
 find_package(PkgConfig)