diff --git a/ports/gsl/CMakeLists.txt b/ports/gsl/CMakeLists.txt
index 3c9c69171d..1efdfcea32 100644
--- a/ports/gsl/CMakeLists.txt
+++ b/ports/gsl/CMakeLists.txt
@@ -41,9 +41,8 @@ endforeach()
 
 file(READ gsl_types.h GSLTYPES_H)
 if(WIN32)
-    string(REPLACE "#ifdef WIN32" "#if 1 /*WIN32*/" GSLTYPES_H "${GSLTYPES_H}")
     if(BUILD_SHARED_LIBS)
-        string(REPLACE "#  ifdef GSL_DLL" "#  if 1 /*GSL_DLL*/" GSLTYPES_H "${GSLTYPES_H}")
+        target_compile_definitions(gsl PRIVATE GSL_DLL)
     endif()
 endif(WIN32)
 file(WRITE ${CMAKE_CURRENT_BINARY_DIR}/gsl_types.h "${GSLTYPES_H}")