Allow the inclusion of an external fmtlib as as sub_directory

CMakeLists.txt

@@ -54,7 +54,7 @@ option(SPDLOG_BUILD_BENCH "Build benchmarks (Requires https://github.com/google/
 option(SPDLOG_BUILD_TESTS "Build tests" ${SPDLOG_MASTER_PROJECT})
 option(SPDLOG_FMT_EXTERNAL "Use external fmt library instead of bundled" OFF)
 
-if(SPDLOG_FMT_EXTERNAL)
+if(SPDLOG_FMT_EXTERNAL AND NOT TARGET fmt::fmt)
     find_package(fmt REQUIRED CONFIG)
 endif()