Make BUILD_TESTS an option
Make BUILD_TESTS an option

This makes
    cmake -DBUILD_TESTS=Yes ..
    cmake -DBUILD_TESTS=YES ..
    cmake -DBUILD_TESTS=On ..
    cmake -DBUILD_TESTS=ON ..
    ...
work as expected (tests will be built). If BUILD_TESTS isn't an option
only cmake -DBUILD_TESTS=Yes .. works.

Signed-off-by: Luís Pereira <95d1463b1ff9eef535e39fb0d755bc242cfcea61@gmail.com>

lxde/libqtxdg,gilir/libqtxdg-qt5,gilir/libqtxdg-qt5,luis-pereira/libqtxdg,lxde/libqtxdg,DrTobe/libqtxdg,luis-pereira/libqtxdg,libqtxdg/libqtxdg,DrTobe/libqtxdg,libqtxdg/libqtxdg,DrTobe/libqtxdg