Set CMAKE_OSX_DEPLOYMENT_TARGET

This version represents that current reality although we
would like to lower this for increased compatibility. See: #4299

CMakeLists.txt

@@ -1,6 +1,12 @@
 # Version set according the the cmake versions available in Ubuntu/Bionic:
 # https://packages.ubuntu.com/bionic/cmake
 cmake_minimum_required(VERSION 3.10.2)
+
+# Needed for C++17 (std::variant)
+# TODO(https://github.com/WebAssembly/binaryen/issues/4299): We need
+# to reduce this for compatability with emsdk.
+set(CMAKE_OSX_DEPLOYMENT_TARGET "10.14" CACHE STRING "Minimum OS X deployment version")
+
 project(binaryen LANGUAGES C CXX VERSION 102)
 include(GNUInstallDirs)