The error was:
c:\projects\opus\celt\celt_encoder.c(1019): fatal error C1001: An internal error has occurred in the compiler. [C:\projects\opus\win32\VS2015\opus.vcxproj]
  (compiler file 'f:\dd\vctools\compiler\utc\src\p2\main.c', line 255)
   To work around this problem, try simplifying or changing the program near the locations listed above.
  Please choose the Technical Support command on the Visual C++
   Help menu, or open the Technical Support help file for more information

  Internal Compiler Error in C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\bin\x86_amd64\CL.exe.  You will be prompted to send an error report to Microsoft later.