CMake: for shared library builds, link curl to its dependencies with …

…PRIVATE

The current PUBLIC visibility causes issues for downstream users.
Cf OSGeo/PROJ#3172 (comment)

lib/CMakeLists.txt

@@ -95,7 +95,14 @@ if(NOT BUILD_SHARED_LIBS)
     set_target_properties(${LIB_NAME} PROPERTIES INTERFACE_COMPILE_DEFINITIONS CURL_STATICLIB)
 endif()
 
-target_link_libraries(${LIB_NAME} ${CURL_LIBS})
+if(BUILD_SHARED_LIBS)
+    target_link_libraries(${LIB_NAME} PRIVATE ${CURL_LIBS})
+else()
+    # It is arguable that the default PUBLIC visibility is correct for
+    # static builds, but it might be a convenient workaround for now,
+    # as correct dependency tracking for static build requires more effort.
+    target_link_libraries(${LIB_NAME} ${CURL_LIBS})
+endif()
 
 set_target_properties(${LIB_NAME} PROPERTIES
   COMPILE_DEFINITIONS BUILDING_LIBCURL