Fix RPM packaging on cmake 3.18+

cmake has caught up and since version 3.18 it started supporting
CPACK_RPM_POST_TRANS_SCRIPT_FILE, something we've supported for
two years and cmake 2.8.11. Both implementation add %posttrans tag
and rpmbuild gets confused.

Disable our implementation for cmake 3.18+

cmake/Internal/CPack/CPackRPM.cmake

@@ -26,26 +26,28 @@ set_from_component(VENDOR)
 # the scriptlet, if present, is appended (together with the %posttrans tag)
 # to the pre-uninstall scriptlet
 #
-set(base_time "PRE")
-set(base_type "UNINSTALL")
-set(base_var CPACK_RPM_${CPACK_RPM_PACKAGE_COMPONENT}_${base_time}_${base_type}_SCRIPT_FILE)
-set(acc)
+if(CMAKE_VERSION VERSION_LESS 3.18)
+  set(base_time "PRE")
+  set(base_type "UNINSTALL")
+  set(base_var CPACK_RPM_${CPACK_RPM_PACKAGE_COMPONENT}_${base_time}_${base_type}_SCRIPT_FILE)
+  set(acc)
 
-macro(read_one_file time_ type_ tag_)
+  macro(read_one_file time_ type_ tag_)
     set(var CPACK_RPM_${CPACK_RPM_PACKAGE_COMPONENT}_${time_}_${type_}_SCRIPT_FILE)
     if (${var})
       file(READ ${${var}} content)
       set(acc "${tag_}\n${content}\n\n${acc}")
     endif()
-endmacro()
+  endmacro()
 
-read_one_file("POST" "TRANS" "%posttrans")
-if (acc)
+  read_one_file("POST" "TRANS" "%posttrans")
+  if (acc)
     set(orig_${base_var} ${${base_var}})
     read_one_file(${base_time} ${base_type} "")
     set(${base_var} ${CPACK_TOPLEVEL_DIRECTORY}/SPECS/${CPACK_RPM_PACKAGE_COMPONENT}_${base_time}_${base_type}.scriptlet)
     file(WRITE ${${base_var}} "${acc}")
-endif()
+  endif()
+endif(CMAKE_VERSION VERSION_LESS 3.18)
 
 # load the original CPackRPM.cmake
 set(orig_CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH})
@@ -59,7 +61,9 @@ set(CMAKE_MODULE_PATH ${orig_CMAKE_MODULE_PATH})
 
 restore(LICENSE)
 restore(VENDOR)
-set(${base_var} ${orig_${base_var}})
+if(${orig_${base_var}})
+  set(${base_var} ${orig_${base_var}})
+endif()
 
 # per-component cleanup
 foreach(_RPM_SPEC_HEADER URL REQUIRES SUGGESTS PROVIDES OBSOLETES PREFIX CONFLICTS AUTOPROV AUTOREQ AUTOREQPROV)