Implement a configure option to select which modules to configure in a top level build

The top level build system has the QT_BUILD_SUBMODULES variable which can be set at configure time to select which modules the configure run should include. E.g.

$ cd dev-small-build
$ ../dev/configure -developer-build -- -DQT_BUILD_SUBMODULES=qtdeclarative

Since QT_BUILD_SUBMODULES is poorly named (this is a configure setting, not a build setting; what modules and targets to build is specified when calling ninja), and since building only a subset of a top level source tree a common thing to do, it would be good to have proper command line support for this variable, similar to the -skip switch:

$ cd dev-small-build
$ ../dev/configure -developer-build -submodules=qtdeclarative

including some documentation

-submodules <repo> ......... Include the listed repositories from the build.

Alternatives to the name of the option:

• -repos or -repositories?
• -modules - not great because the modules are called Core, Widgets, Quick