Resolve chicken-egg-problem with extconf.h by adding the include path

without check.

ext/fox16_c/extconf.rb

@@ -342,8 +342,8 @@ def do_rake_compiler_setup
   find_library("GLU", "gluNewQuadric", "/usr/X11R6/lib")
   $libs = append_library($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")
-  find_header('FXRbCommon.h', File.join(File.dirname(__FILE__), 'include'))
   $libs = append_library($libs, "FOX-1.6")
+  $INCFLAGS << " -I#{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')
     $CPPFLAGS = $CPPFLAGS + " -DWITH_FXSCINTILLA -DHAVE_FOX_1_6"