Index: Modules/Compiler/GNU-Fortran.cmake
--- Modules/Compiler/GNU-Fortran.cmake.orig
+++ Modules/Compiler/GNU-Fortran.cmake
@@ -17,9 +17,11 @@ endif()
 
 set(CMAKE_Fortran_POSTPROCESS_FLAG "-fpreprocessed")
 
-# No -DNDEBUG for Fortran.
-string(APPEND CMAKE_Fortran_FLAGS_MINSIZEREL_INIT " -Os")
-string(APPEND CMAKE_Fortran_FLAGS_RELEASE_INIT " -O3")
+if(NOT DEFINED ENV{MODCMAKE_PORT_BUILD})
+  # No -DNDEBUG for Fortran.
+  string(APPEND CMAKE_Fortran_FLAGS_MINSIZEREL_INIT " -Os")
+  string(APPEND CMAKE_Fortran_FLAGS_RELEASE_INIT " -O3")
+endif()
 
 # No -isystem for Fortran because it will not find .mod files.
 unset(CMAKE_INCLUDE_SYSTEM_FLAG_Fortran)