update readme added build failure soluction according to my own experience
@@ -26,6 +26,16 @@ source .travis/s2n_setup_env.sh
.travis/s2n_travis_build.sh
```
+* For OSX 10 Mojave users, if you are facing ```Undefined symbols for architecture x86_64:``` error in building. Starting with Mojave, the headers are no longer installed under ```/usr/include``` by default -- look under ```Command Line Tools -> New Features``` in the [release notes](https://developer.apple.com/documentation/xcode_release_notes/xcode_10_release_notes).
+
+ Running
+
+ ```open /Library/Developer/CommandLineTools/Packages/macOS_SDK_headers_for_macOS_10.14.pkg```
+
+ from the command line fixes the issue.
+
+ Thanks for solution provider [@clason](https://github.com/clason), whose original answer could be found [here](https://github.com/neovim/neovim/issues/9050#issuecomment-424417456).
+
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