Steps to reproduce:
1. Download "TestUnityBug.zip" provided by the user
2. Extract and open the project
3. Go to build settings, make sure the build target is PC, MAC & Linux standalone
3.1. If it's not, switch platform to standalone, exit build settings and open them again
4. Switch platform to iOS
5. Build the project
6. Once the build finishes, observe the console

Expected result: console outputs lines
buildTarget=iOS
UNITY_IOS defined

Actual result: console outputs lines
buildTarget=iOS
UNITY_STANDALONE defined

Workaround: after switching platform, click on the main Unity window, then then preprocessor directives change accordingly
Reproduced on: 5.4.4p3, 5.5.1p4
Not reproduced on: 5.6.0b9