-skip-tests and -skip-examples are case sensitive

When performing a top-level build, the `-skip-tests` and `-skip-examples` are very useful options, but they are case-sensitive which does not match the behavior of other common flags like `-submodules`. This can lead to confusion because the build system does not check that there is a submodule with the given name, and it is easy to miss the "Manually-specified variables were not used by the project" warning at the end of configure output.

For example, passing `-skip-tests qtdeclarative`, at the end of configure I see

Manually-specified variables were not used by the project:

 QT_BUILD_TESTS_PROJECT_qtdeclarative