Don't know if you worked this out yet or not but

I just compiled 5.5.1 on VS 2015 on x64 Windows 10. today I was following this guide http://doc.qt.io/qt-5/windows-building.html -- maybe this will help you or someone.

my qt5vars.cmd file looked like this:

Qt Code:
  1. REM Set up \Microsoft Visual Studio 2013, where <arch> is \c amd64, \c x86, etc.
  2. CALL "C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\vcvarsall.bat" x64
  3. SET _ROOT=C:\qt\qt-5.5.1
  4. echo --ROOT
  5. echo %_ROOT%
  6. SET PATH=%_ROOT%\qtbase\bin;%_ROOT%\gnuwin32\bin;C:\Python27;C:\Perl64;C:\Ruby22-x64;%PATH%
  7. REM Uncomment the below line when using a git checkout of the source repository
  8. REM SET PATH=%_ROOT%\qtrepotools\bin;%PATH%
  9. SET QMAKESPEC=win32-msvc2015
  10. echo --QMAKESPEC
  11. echo %QMAKESPEC%
  12. SET _ROOT=
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then typed<
configure -debug -nomake examples -opensource

Also I used Jom instead of nmake definitely sped up the process just drop the jom files in your root folder to use it.