[build] Generate the LICENSE.md for mbgl-core

Add a script that for everything we link against
mbgl-core, it will check the license and generate
a file all the dependencies listed with the respective
authors, project urls and license.

scripts/license.cmake

@@ -0,0 +1,40 @@
+# Generate the license file for the target ${param} at config time
+
+function(mbgl_generate_license param)
+    # Fake targets or non relevant.
+    set(BLACKLIST "mbgl-compiler-options")
+
+    get_target_property(LIBRARIES ${param} LINK_LIBRARIES)
+    list(INSERT LIBRARIES 0 ${param})
+
+    # cmake-format: off
+    foreach(LIBRARY IN LISTS LIBRARIES)
+    # cmake-format: on
+        if(${LIBRARY} IN_LIST BLACKLIST)
+            continue()
+        endif()
+
+        if(TARGET ${LIBRARY})
+            get_target_property(NAME ${LIBRARY} INTERFACE_MAPBOX_NAME)
+            get_target_property(URL ${LIBRARY} INTERFACE_MAPBOX_URL)
+            get_target_property(AUTHOR ${LIBRARY} INTERFACE_MAPBOX_AUTHOR)
+            get_target_property(LICENSE ${LIBRARY} INTERFACE_MAPBOX_LICENSE)
+
+            if(NOT LICENSE OR NOT EXISTS ${LICENSE})
+                message(FATAL_ERROR "License not found for target: ${LIBRARY}")
+            endif()
+
+            file(READ ${LICENSE} LICENSE_DATA)
+
+            string(APPEND LICENSE_LIST "### [${NAME}](${URL}) by ${AUTHOR}\n\n")
+            string(APPEND LICENSE_LIST "```\n${LICENSE_DATA}\n```\n\n")
+            string(APPEND LICENSE_LIST "---\n\n")
+        endif()
+    endforeach()
+
+    file(WRITE ${CMAKE_BINARY_DIR}/${param}.license ${LICENSE_LIST})
+
+    add_custom_target(${param}-license COMMAND cat ${CMAKE_BINARY_DIR}/${param}.license)
+endfunction()
+
+mbgl_generate_license(mbgl-core)