I'm building on MS-Win (to make sense of a CI failure), doing an out-of-source (a.k.a. "shadow") build.
I start by running configure.bat; this builds configure.exe in my build directory.
When I try to run that, however, I get:

Unable to detect the platform from environment. Use -platform command lineargument or set the QMAKESPEC environment variable and run configure again
See the README file for a list of supported operating systems and compilers.

There is no README file.

The first line of the error message needs a '\n' between "line" and "argument" (and a full stop at the end, after "again").
The last line should refer to somewhere that actually exists, to let the user find out what it might make sense to pass to -platform.

On
http://doc.qt.io/qt-5/configure-options.html#compiler-options
I find a claim that win32-g++ and win32-msvc2010 are valid options; however, both are rejected by my actual (freshly-built) configure.exe:

MS-Tester@MSware MINGW64 ~/build/qtbase
$ ./configure.exe -platform win32-g++ -developer-build -debug -opensource -confirm-license -no-compile-examples
Invalid option "win32-g++" for -platform.
See the README file for a list of supported operating systems and compilers.

MS-Tester@MSware MINGW64 ~/build/qtbase
$ ./configure.exe -platform win32-msvc2015 -help
Invalid option "win32-msvc2015" for -platform.
See the README file for a list of supported operating systems and compilers.

MS-Tester@MSware MINGW64 ~/build/qtbase
$ ./configure.exe -platform win32-msvc2010 -help
Invalid option "win32-msvc2010" for -platform.
See the README file for a list of supported operating systems and compilers.

I have both MSVC 2015 (albeit without Visual Studio) and MinGW installed, so both compilers are available and in my PATH.

The -help option should be capable of working without a valid -platform specified - and it should provide a list of what -platform values have some chance of working.