'Building' qtDoc: NMAKE : fatal error U1095: expanded command line ... too long

In `qtDoc\doc\doc.pri` it builds up a huuuge command like the following:

online_docs:
set QT_CORE_SOURCES=qtSrc\5.0\qtBase\src\corelib &&
set QT_DECLARATIVE_SOURCES=qtSrc\5.0\qtDeclarative\src\declarative &&
set QT_DOC_SOURCES=qtSrc\5.0\qtDoc
&& ... &&
qtBuild\5.0\bin\qdoc.exe qtSrc\5.0\qtDoc\doc\config\qt-build-docs-online.qdocconf

This seems to assume that nmake or windows shell have no command length limitations. Sadly this is not the case.

So instead of generating one massive command in the Makefile, with (&&) why not just separate them out into separate ones...

My fix is attached