diff --git a/CMakeLists.txt b/CMakeLists.txt
index 83b0186..dfe1704 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -90,10 +90,6 @@ endif()
 
 check_symbol_exists(open_memstream "stdio.h" HAVE_OPEN_MEMSTREAM)
 
-if (HAVE_OPEN_MEMSTREAM)
-    extra_cxx_compiler_flag("-DHAVE_OPEN_MEMSTREAM=1")
-endif()
-
 cmake_push_check_state()
 set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} ${CUPS_CFLAGS}")
 check_include_file_cxx(cups/raster.h HAVE_CUPS_RASTER_H)
diff --git a/config.h.in b/config.h.in
index 601ac5e..755f450 100644
--- a/config.h.in
+++ b/config.h.in
@@ -1,2 +1,4 @@
 #define PACKAGE "@CMAKE_PROJECT_NAME@"
 #define VERSION "@BRLASER_VERSION@"
+
+#cmakedefine HAVE_OPEN_MEMSTREAM
diff --git a/test/tempfile.h b/test/tempfile.h
index 45831fb..b79856c 100644
--- a/test/tempfile.h
+++ b/test/tempfile.h
@@ -21,6 +21,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <vector>
+#include "config.h"
 
 class tempfile {
  public: