Override topdir from rpm command line seems necessary on Amazon linux.

This modification should be harmless on other distro.
Signed-off-by: Eric NOULARD <eric.noulard@gmail.com>

Modules/CPackRPM.cmake

@@ -899,6 +899,7 @@ IF(RPMBUILD_EXECUTABLE)
   # Now call rpmbuild using the SPECFILE
   EXECUTE_PROCESS(
     COMMAND "${RPMBUILD_EXECUTABLE}" -bb
+            --define "_topdir ${CPACK_RPM_DIRECTORY}"
             --buildroot "${CPACK_RPM_DIRECTORY}/${CPACK_PACKAGE_FILE_NAME}${CPACK_RPM_PACKAGE_COMPONENT_PART_PATH}"
             "${CPACK_RPM_BINARY_SPECFILE}"
     WORKING_DIRECTORY "${CPACK_TOPLEVEL_DIRECTORY}/${CPACK_PACKAGE_FILE_NAME}${CPACK_RPM_PACKAGE_COMPONENT_PART_PATH}"