Replace STRING(PREPEND ..) in CMakeLists.txt to support older CMake versions

CMakeLists.txt

@@ -205,7 +205,7 @@ if (SPDLOG_INSTALL)
     #---------------------------------------------------------------------------------------
     get_target_property(PKG_CONFIG_DEFINES spdlog INTERFACE_COMPILE_DEFINITIONS)
     string(REPLACE ";" " -D" PKG_CONFIG_DEFINES "${PKG_CONFIG_DEFINES}")
-    string(PREPEND PKG_CONFIG_DEFINES "-D")
+    string(CONCAT PKG_CONFIG_DEFINES "-D" "${PKG_CONFIG_DEFINES}")
     configure_file("cmake/${PROJECT_NAME}.pc.in" "${pkg_config}" @ONLY)
     install(FILES "${pkg_config}" DESTINATION "${pkgconfig_install_dir}")