Emit deprecation warning on unscoped import target usage

CMake will print the following warning:

CMake Warning (dev) at CMakeLists.txt:n (target_link_libraries):
  The library that is being linked to, ceres, is marked as being deprecated
  by the owner.  The message provided by the developer is:

  The unscoped import target `ceres' is deprecated and will be removed in a
  future Ceres Solver release.  Please use the Ceres::ceres import target
  instead.

This warning is for project developers.  Use -Wno-dev to suppress it.

Change-Id: I5abc875b1f61e1f47523375d1579a188047c45d4

cmake/CeresConfig.cmake.in

diff --git a/cmake/CeresConfig.cmake.in b/cmake/CeresConfig.cmake.in
index 9396d84..8b44612 100644
--- a/cmake/CeresConfig.cmake.in
+++ b/cmake/CeresConfig.cmake.in
@@ -310,4 +310,10 @@
   # true ALIAS library in CMake terms as they cannot point to imported targets.
   add_library(ceres INTERFACE IMPORTED)
   set_target_properties(ceres PROPERTIES INTERFACE_LINK_LIBRARIES Ceres::ceres)
+
+  if (CMAKE_VERSION VERSION_GREATER_EQUAL 3.17)
+    set_target_properties(ceres PROPERTIES DEPRECATION
+"The unscoped import target `ceres' is deprecated and will be removed in a \
+future Ceres Solver release. Please use the Ceres::ceres import target instead.")
+  endif (CMAKE_VERSION VERSION_GREATER_EQUAL 3.17)
 endif()