Resolving the Unresolved reference to symbol 'CustomAction:ConfigureIIs'

This error occurs when you are building your MSI using light.exe and are getting the following
error LGHT0112 : Unresolved reference to symbol 'CustomAction:ConfigureIIs' in section....

When you look for the symbol in your .wxs, you dont find it there. Imagine it like not having a reference to a Dll which your .net code requires and you need to link a library. If your .wxs is modifying any websites or virtual directories, you need to link the appropriate wix library (its of type .wixlib). For Website and virtual directories, the library you need to include is sca.wixlib.

So your build command should be:
light.exe sca.wixlib SomeWxs.wixobj -out SomeMSI.msi

Tip:- Its important to give -out when you are giving more than 1 file as input to light.exe.
Other common libraries are: wixca.wixlib, wixui.wixlib.