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]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

@@           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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[ntadej]

Thanks for your contribution!

In principle I am fine with the changes, but probably we can clean the include paths a bit more. I can also do it later if you want.