diff --git a/CMakeLists.txt b/CMakeLists.txt
index f303be4e..14a091e1 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -41,6 +41,10 @@ if(APPLE)
 	# the / is required by some older versions of OS X
 	set(CMAKE_INSTALL_RPATH "@executable_path/")
 	set(CMAKE_MACOSX_RPATH TRUE)
+
+	# older versions of cmake incorrectly treat Objective-C as C++
+	# see also https://cmake.org/pipermail/cmake-developers/2014-March/010072.html
+	list(APPEND CMAKE_C_SOURCE_FILE_EXTENSIONS m)
 elseif(WIN32)
 	set(_OSNAME windows)