I have been successfully using Qt 5.8 (from qt-opensource-windows-x86-msvc2015-5.8.0.exe) for several years.
I fear that later versions of Qt will not become available in the future. For example, www.qt.io/download no longer contains links to pre-compiled versions (such as qt-opensource-windows-x86-msvc2015-5.8.0.exe)
So, I decided to attempt to build Qt from source on Windows.
I have successfully built many versions of Qt on Linux in the past, but I have never been able to build Qt on Windows.

I am NOT able to build and install Qt 5.11.2 open source on Windows/MSVC 2015 due to problems with the QWebEngine compile failing. The QWebEngine compile absolutely requires a 64bit linker.
So after much research I came to the conclusion that I needed to obtain MSVC2017 to get a build to work.

Through much trial and error I found that the source and destination paths MUST be very short or I get "Path too long" build errors.
But, I now have been able to build and install Qt 5.11.2 open source on Windows/MSVC 2017 using a 64_86 Visual Studio command prompt.

I am able to create a new GUI project that builds and runs with it.
But although I can also compile my huge existing code base, none of the applications run due to problems with DLL's (they load and immediately unload)
I suspect problems with 64 vs 32 bit exec/dlls but cannot find any obvious issue. Depends shows several Windows 64 bit DLLs.

NOTE: I require 32 bit executables due to hardware vendors only providing 32 bit driver interfaces.

Here is the configure command used (where BUILD_INSTALL_DIR is 5.11.2:

..\qt5\configure -prefix C:\Qt\%BUILD_INSTALL_DIR% -verbose -debug-and-release -opensource -confirm-license -icu -opengl desktop -shared -plugin-sql-mysql -nomake examples -nomake tests -platform win32-msvc2017 -openssl-linked OPENSSL_LIBS="-lssleay32 -llibeay32" -I C:\openssl\include -L C:\openssl\lib -I C:\MySQL\include -L C:\MySQL\lib ICU_PREFIX=C:\ICU -I C:\ICU\include -L C:\ICU\lib

Are there new requirements for the project files? I looked at the test GUI project file and did not see anything special that is not in my project files.

Any suggestions for further investigation would be appreciated