diff --git a/CMakeLists.txt b/CMakeLists.txt
index 44b77de48..625ebb334 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -82,6 +82,13 @@ if(WIN32)
     endif()
 endif()
 
+if (APPLE)
+    if (NOT CMAKE_OSX_DEPLOYMENT_TARGET)
+        set(CMAKE_OSX_DEPLOYMENT_TARGET "10.9" CACHE STRING "OS X Deployment target (SDK version)" FORCE)
+    endif ()
+    message(STATUS "Mac OS deployment target (SDK version): ${CMAKE_OSX_DEPLOYMENT_TARGET}")
+endif ()
+
 if (CMAKE_SYSTEM_NAME STREQUAL "Linux")
     # Workaround for an old CMake, which does not understand CMAKE_CXX_STANDARD.
     add_compile_options(-std=c++11 -Wall -Wno-reorder)
diff --git a/deps/deps-macos.cmake b/deps/deps-macos.cmake
index 10c15dace..720ec50d0 100644
--- a/deps/deps-macos.cmake
+++ b/deps/deps-macos.cmake
@@ -23,8 +23,8 @@ ExternalProject_Add(dep_boost
         variant=release
         threading=multi
         boost.locale.icu=off
-        "cflags=cflags=-fPIC -mmacosx-version-min=${DEPS_OSX_TARGET}"
-        "cxxflags=cxxflags=-fPIC -mmacosx-version-min=${DEPS_OSX_TARGET}"
+        "cflags=-fPIC -mmacosx-version-min=${DEPS_OSX_TARGET}"
+        "cxxflags=-fPIC -mmacosx-version-min=${DEPS_OSX_TARGET}"
         install
     INSTALL_COMMAND ""   # b2 does that already
 )