Downgrade OSX deployment target to 10.12 (WebAssembly#308)

This change switches the MacOS build to target 10.12, an older version
than currently supported. This would close WebAssembly#127. @sbc100 mentions that
this is the setting used by Emscripten and related projects.

Author: abrown

Makefile

@@ -62,7 +62,7 @@ build/llvm.BUILT:
 		-DLLVM_STATIC_LINK_CXX_STDLIB=ON \
 		-DLLVM_HAVE_LIBXAR=OFF \
 		-DCMAKE_OSX_ARCHITECTURES="arm64;x86_64" \
-		-DCMAKE_OSX_DEPLOYMENT_TARGET=11.0 \
+		-DCMAKE_OSX_DEPLOYMENT_TARGET=10.12 \
 		-DCMAKE_INSTALL_PREFIX=$(PREFIX) \
 		-DLLVM_TARGETS_TO_BUILD=WebAssembly \
 		-DLLVM_DEFAULT_TARGET_TRIPLE=wasm32-wasi \