cmake: enable CMP0051

the new behavior of get_target_property(<var> <target> SOURCES) will be
enforced in future versions of cmake, so let ready for this change now
by removing the generator expressions from the returned source file
list.

Signed-off-by: Kefu Chai <[email protected]>

CMakeLists.txt

@@ -19,13 +19,7 @@ if(POLICY CMP0065)
   cmake_policy(SET CMP0065 NEW)
 endif()
 if(POLICY CMP0051)
-  # cmake 3.1 and higher include generator expressions in SOURCES property.
-  # in BuildBoost.cmake, get_target_property(<var> <target> SOURCES) is used
-  # to retrieve the source files of a target. in that case, we are only
-  # interested in the *source* files. and i don't want to bother stripping off
-  # the TARGET_OBJECTS elements from the returned SOURCES. so let's stick with
-  # the old behavior now.
-  cmake_policy(SET CMP0051 OLD)
+  cmake_policy(SET CMP0051 NEW)
 endif()
 list(APPEND CMAKE_MODULE_PATH "${CMAKE_SOURCE_DIR}/cmake/modules/")
 

cmake/modules/BuildBoost.cmake

@@ -254,6 +254,9 @@ function(maybe_add_boost_dep target)
     return()
   endif()
   get_target_property(sources ${target} SOURCES)
+  if(NOT CMAKE_VERSION VERSION_LESS 3.1)
+    string(GENEX_STRIP "${sources}" sources)
+  endif()
   foreach(src ${sources})
     get_filename_component(ext ${src} EXT)
     # assuming all cxx source files include boost header(s)