diff --git a/build-support/macros-and-definitions.cmake b/build-support/macros-and-definitions.cmake
index c90ea5b4a..d18a96c89 100644
--- a/build-support/macros-and-definitions.cmake
+++ b/build-support/macros-and-definitions.cmake
@@ -796,6 +796,7 @@ macro(process_options)
       message(STATUS "GSL was not found. Continuing without it.")
     else()
       message(STATUS "GSL was found.")
+      add_definitions(-DHAVE_GSL)
     endif()
   endif()