cmake: do not use CMP0093 unless it is supported

CMake CMP0093 was introduced by CMake 3.15, but we only require CMake
v3.10.2. so use the old `Boost_VERSION_MACRO` variable by default,
unless specified otherwise.

Signed-off-by: Kefu Chai <kchai@redhat.com>
(cherry picked from commit f5b2ec5d8fd7ce754b0a94d62d0e074269fbb0b5)

diff --git a/cmake/modules/FindBoost.cmake b/cmake/modules/FindBoost.cmake
index 001d7cfa48e8b63684dde560d65d7fb085723773..29aa2c32e2367f755504222966024dcbc02468cf 100644 (file)
--- a/cmake/modules/FindBoost.cmake
+++ b/cmake/modules/FindBoost.cmake
@@ -1649,15 +1649,17 @@ if(Boost_INCLUDE_DIR)
   set(Boost_VERSION_STRING "${Boost_VERSION_MAJOR}.${Boost_VERSION_MINOR}.${Boost_VERSION_PATCH}")
 
   # Define final Boost_VERSION
-  cmake_policy(GET CMP0093 _Boost_CMP0093
-    PARENT_SCOPE # undocumented, do not use outside of CMake
-  )
-  if("x${_Boost_CMP0093}x" STREQUAL "xNEWx")
-    set(Boost_VERSION ${Boost_VERSION_STRING})
+  if(POLICY CMP0093)
+    cmake_policy(GET CMP0093 _Boost_CMP0093)
+    if("x${_Boost_CMP0093}x" STREQUAL "xNEWx")
+      set(Boost_VERSION ${Boost_VERSION_STRING})
+    else()
+      set(Boost_VERSION ${Boost_VERSION_MACRO})
+    endif()
+    unset(_Boost_CMP0093)
   else()
     set(Boost_VERSION ${Boost_VERSION_MACRO})
   endif()
-  unset(_Boost_CMP0093)
 
   _Boost_DEBUG_PRINT_VAR("${CMAKE_CURRENT_LIST_FILE}" "${CMAKE_CURRENT_LIST_LINE}" "Boost_VERSION")
   _Boost_DEBUG_PRINT_VAR("${CMAKE_CURRENT_LIST_FILE}" "${CMAKE_CURRENT_LIST_LINE}" "Boost_VERSION_STRING")