Define the deployment target in the frameworks metadata (#425)

Summary:
Fixes #420

Apple App Store submission, at least the iOS one, requires this metadata to be specified when only targeting a subset of possible archs.

Pull Request resolved: #425

Test Plan:
(Taken from #428)

```
~/tmp » curl -L -O https://12725-154201259-gh.circle-artifacts.com/0/tmp/hermes/output/hermes-engine-darwin-v0.7.1.tgz
~/tmp » tar -zvxf hermes-engine-darwin-v0.7.1.tgz
~/tmp » cd package/
~/t/package » node unpack-tarball.js
~/t/package » /usr/libexec/PlistBuddy -c 'Print :MinimumOSVersion' destroot/Library/Frameworks/iphoneos/hermes.framework/Info.plist
10.0
~/t/package » /usr/libexec/PlistBuddy -c 'Print :LSMinimumSystemVersion' destroot/Library/Frameworks/macosx/hermes.framework/Resources/Info.plist
10.13
```

Reviewed By: mhorowitz

Differential Revision: D25839408

Pulled By: Huxpro

fbshipit-source-id: 8067069d66ea3af36366157ffae95445e9e89392

API/hermes/CMakeLists.txt

@@ -129,6 +129,16 @@ if(APPLE AND HERMES_BUILD_APPLE_FRAMEWORK)
     target_compile_options(libhermes PUBLIC "-fembed-bitcode")
     target_link_libraries(libhermes PUBLIC "-fembed-bitcode")
   endif()
+  # Define the deployment target in the frameworks metadata
+  if(HERMES_APPLE_TARGET_PLATFORM MATCHES "iphone")
+    add_custom_command(TARGET libhermes POST_BUILD
+      COMMAND /usr/libexec/PlistBuddy -c "Add :MinimumOSVersion string ${CMAKE_OSX_DEPLOYMENT_TARGET}" $<TARGET_FILE_DIR:libhermes>/Info.plist
+    )
+  elseif(HERMES_APPLE_TARGET_PLATFORM MATCHES "macos")
+    add_custom_command(TARGET libhermes POST_BUILD
+      COMMAND /usr/libexec/PlistBuddy -c "Add :LSMinimumSystemVersion string ${CMAKE_OSX_DEPLOYMENT_TARGET}" $<TARGET_FILE_DIR:libhermes>/Resources/Info.plist
+    )
+  endif()
 endif()
 
 install(TARGETS libhermes