Detect and report the "missing DLL for host tools" situation when building Qt

When building Qt on Windows with non-bundled 3rdparty libraries, it's a common mistake to not have the 3rdparty lib's DLLs in PATH. QTBUG-113139 is an example.

This is an user error that seems to be a bit tough to find. Qt's build system could detect this situation and provide a hint.

For all host tools like rcc and friends we could provide a wrapper batch file (like in Qt5 times) that does the following check after calling the tool:

if "%ERRORLEVEL%" == "-1073741515" echo DLL not found.

Instead of "DLL not found." we would of course give an explanation about the likely cause of this error.

As an alternative to the wrapper batch file, we could try to run each host tool after it has been built.