Add export targets to be able to use find_package() on build folder.

CMakeLists.txt

@@ -56,6 +56,28 @@ install(
    COMPONENT Development
    DESTINATION ${CMAKE_INSTALL_LIBDIR}/cmake/${PROJECT_NAME} )
 
+
+
+# Export targets from the *build* tree for use by outside projects.
+#
+# INFO:
+# Thus, this project can be used by other projects without being installed.
+# This is useful for development: you can build a child projet without an
+# install path.
+# For example:
+# while generating GeoModel projects with CMake as Xcode project on macOS,
+# after having built GeoModelCore, you can use that to build GeoModelIO without
+# the need of an installation step, by simply prepend 'GeoModelCore_DIR' as this:
+#     $ GeoModelCore_DIR=../b_core/CMakeFiles cmake -G Xcode ../../GeoModelIO
+# which is useful when generating projects
+# that needs to be built with Xcode. Xcode in fact does not handle well the install
+# folder generated by CMake: sometimes the content of that is not updated when
+# updating the code, which results in keeping running the same old code even if
+# we modified our source code.
+# export(EXPORT ${PROJECT_NAME}-export NAMESPACE "${PROJECT_NAME}::" FILE ${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/${PROJECT_NAME}Config.cmake)
+export(TARGETS GeoGenericFunctions GeoModelKernel NAMESPACE "${PROJECT_NAME}::" FILE ${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/${PROJECT_NAME}Targets.cmake)
+
+
 # Set up the packaging of the project using CPack.
 include( GeoModelCorePackaging )
 
@@ -79,9 +101,10 @@ if (DOXYGEN_FOUND)
 
   # request to configure the file
   configure_file(${DOXYGEN_IN} ${DOXYGEN_OUT} @ONLY)
-  message("Doxygen build started")
 
-  # note the option ALL which allows to build the docs together with the application
+  # note:
+  # - `make doc` builds the Doxygen docs on request
+  # - `make all` allows to build the docs together with the application
   add_custom_target( doc
       COMMAND ${DOXYGEN_EXECUTABLE} ${DOXYGEN_OUT}
       WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}