Running windows/build.cmd from the folder C:/Users/Andrew Dawson/Projects/libcec fails to build. My username has a space in it. Running from C:/libcec works fine for the moment. So that's the workaround.
libcec-support/windows/cmake/build.cmd also fails to call vcvars64.bat because the full path on my system is C:/Program Files (x86)/Microsoft Visual Studio/BuildTools/VC/Auxiliary/Build/vcvars64.bat. This has both spaces and a closing parentheses ) which is interpreted as the end of an IF statement.