Adding system includes option

CMakeLists.txt

@@ -82,6 +82,7 @@ option(SPDLOG_SANITIZE_ADDRESS "Enable address sanitizer in tests" OFF)
 option(SPDLOG_BUILD_WARNINGS "Enable compiler warnings" OFF)
 
 # install options
+option(SPDLOG_SYSTEM_INCLUDES "Include as system headers (skip for clang-tidy)." OFF)
 option(SPDLOG_INSTALL "Generate the install target" ${SPDLOG_MASTER_PROJECT})
 option(SPDLOG_USE_STD_FORMAT "Use std::format instead of fmt library." OFF)
 option(SPDLOG_FMT_EXTERNAL "Use external fmt library instead of bundled" OFF)
@@ -170,8 +171,16 @@ endif()
 
 add_library(spdlog::spdlog ALIAS spdlog)
 
+set(SPDLOG_INCLUDES_LEVEL "PUBLIC")
+set(SPDLOG_HEADER_INCLUDES_LEVEL "")
+if(SPDLOG_SYSTEM_INCLUDES)
+    set(SPDLOG_INCLUDES_LEVEL "SYSTEM")
+    set(SPDLOG_HEADER_INCLUDES_LEVEL "SYSTEM")
+endif()
+
+
 target_compile_definitions(spdlog PUBLIC SPDLOG_COMPILED_LIB)
-target_include_directories(spdlog PUBLIC "$<BUILD_INTERFACE:${CMAKE_CURRENT_LIST_DIR}/include>"
+target_include_directories(spdlog ${SPDLOG_INCLUDES_LEVEL} "$<BUILD_INTERFACE:${CMAKE_CURRENT_LIST_DIR}/include>"
                                          "$<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>")
 target_link_libraries(spdlog PUBLIC Threads::Threads)
 spdlog_enable_warnings(spdlog)
@@ -190,7 +199,7 @@ endif()
 add_library(spdlog_header_only INTERFACE)
 add_library(spdlog::spdlog_header_only ALIAS spdlog_header_only)
 
-target_include_directories(spdlog_header_only INTERFACE "$<BUILD_INTERFACE:${CMAKE_CURRENT_LIST_DIR}/include>"
+target_include_directories(spdlog_header_only ${SPDLOG_HEADER_INCLUDES_LEVEL} INTERFACE "$<BUILD_INTERFACE:${CMAKE_CURRENT_LIST_DIR}/include>"
                                                         "$<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>")
 target_link_libraries(spdlog_header_only INTERFACE Threads::Threads)