diff --git a/ext/fox16_c/extconf.rb b/ext/fox16_c/extconf.rb
index 89e9b17d9b8e5d5625713f9125e0da38337e8acc..14d0e50bb4e9a72f4edf4ff525a656e2afb25967 100755
--- a/ext/fox16_c/extconf.rb
+++ b/ext/fox16_c/extconf.rb
@@ -110,10 +110,10 @@ def do_rake_compiler_setup
   $libs = append_library($libs, "Xrandr") unless RUBY_PLATFORM =~ /mingw/ || enable_config("win32-static-build")
   $libs = append_library($libs, "Xcursor") unless RUBY_PLATFORM =~ /mingw/ || enable_config("win32-static-build")
   $libs = append_library($libs, "png")
-  $CFLAGS = $CFLAGS + " -O0 -I#{File.join(File.dirname(__FILE__), 'include')}"
+  find_header('FXRbCommon.h', File.join(File.dirname(__FILE__), 'include'))
   if is_fxscintilla_build?
     FileUtils.move('scintilla_wrap.cpp.bak', 'scintilla_wrap.cpp') if FileTest.exist?('scintilla_wrap.cpp.bak')
-    $CFLAGS = $CFLAGS + " -DWITH_FXSCINTILLA -DHAVE_FOX_1_6"
+    $CPPFLAGS = $CPPFLAGS + " -DWITH_FXSCINTILLA -DHAVE_FOX_1_6"
     $libs = append_library($libs, "fxscintilla")
   else
     FileUtils.move('scintilla_wrap.cpp', 'scintilla_wrap.cpp.bak') if FileTest.exist?('scintilla_wrap.cpp')