Index: Modules/Compiler/GNU.cmake
--- Modules/Compiler/GNU.cmake.orig
+++ Modules/Compiler/GNU.cmake
@@ -53,11 +53,19 @@ macro(__compiler_gnu lang)
   endif()
 
   # Initial configuration flags.
-  string(APPEND CMAKE_${lang}_FLAGS_INIT " ")
-  string(APPEND CMAKE_${lang}_FLAGS_DEBUG_INIT " -g")
-  string(APPEND CMAKE_${lang}_FLAGS_MINSIZEREL_INIT " -Os")
-  string(APPEND CMAKE_${lang}_FLAGS_RELEASE_INIT " -O3")
-  string(APPEND CMAKE_${lang}_FLAGS_RELWITHDEBINFO_INIT " -O2 -g")
+  if(DEFINED ENV{MODCMAKE_PORT_BUILD})
+    string(APPEND CMAKE_${lang}_FLAGS_INIT " ")
+    string(APPEND CMAKE_${lang}_FLAGS_DEBUG_INIT " -g")
+    string(APPEND CMAKE_${lang}_FLAGS_MINSIZEREL_INIT " ")
+    string(APPEND CMAKE_${lang}_FLAGS_RELEASE_INIT " ")
+    string(APPEND CMAKE_${lang}_FLAGS_RELWITHDEBINFO_INIT " -g")
+  else()
+    string(APPEND CMAKE_${lang}_FLAGS_INIT " ")
+    string(APPEND CMAKE_${lang}_FLAGS_DEBUG_INIT " -g")
+    string(APPEND CMAKE_${lang}_FLAGS_MINSIZEREL_INIT " -Os")
+    string(APPEND CMAKE_${lang}_FLAGS_RELEASE_INIT " -O3")
+    string(APPEND CMAKE_${lang}_FLAGS_RELWITHDEBINFO_INIT " -O2 -g")
+  endif()
   if(NOT "x${lang}" STREQUAL "xFortran")
     string(APPEND CMAKE_${lang}_FLAGS_MINSIZEREL_INIT " -DNDEBUG")
     string(APPEND CMAKE_${lang}_FLAGS_RELEASE_INIT " -DNDEBUG")