CMake: Added gtl::gtl alias for gtl interface library

[M2-TE]

Having an alias forces CMake to check that it actually exists when e.g. linking to it:

• target_link_libraries(exe PRIVATE gtl) allows CMake to configure without issue, despite gtl missing
• target_link_libraries(exe PRIVATE gtl::gtl) enforces the existance of gtl

[greg7mdp]

Sounds good, thanks for the contribution @M2-TE .

Could you change your new line to add_library(${PROJECT_NAME}::${PROJECT_NAME} ALIAS ${PROJECT_NAME}) and I'll merge it.

[M2-TE]

Woops, midnight oversight. Sorry, done!