Allow to use as a cmake included project

[nunojpg]

With this change I can add this project with "add_subdirectory" and just use it with just QMapLibre::Widgets in the target_link_libraries.

I am only using QT Widgets with set(MLN_QT_WITH_LOCATION OFF), so my patch probably does not fix QML builds.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (64bf8c4) 45.29% compared to head (efab633) 45.29%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #100 +/- ## ======================================= Coverage 45.29% 45.29% ======================================= Files 35 35 Lines 2126 2126 ======================================= Hits 963 963 Misses 1163 1163 ```

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