Fixed an issue with jpeg-turbo library

There were compilation errors if the version of the library was less than 1.2.0.
This was happening because the macros JPEG_LIB_VERSION_MAJOR and JPEG_LIB_VERSION_MINOR were not being defined, since the compile-time macro LIBJPEG_TURBO_VERSION was introduced in version 1.2.0.
The problem was fixed by giving those two macros a definition if it was missing.

src/init.cc

@@ -27,8 +27,10 @@ init (Handle<Object> target) {
   target->Set(String::New("cairoVersion"), String::New(cairo_version_string()));
 #ifdef HAVE_JPEG
 
-#ifdef LIBJPEG_TURBO_VERSION
+#ifndef JPEG_LIB_VERSION_MAJOR
 #define JPEG_LIB_VERSION_MAJOR 8
+#endif
+#ifndef JPEG_LIB_VERSION_MINOR
 #define JPEG_LIB_VERSION_MINOR 4
 #endif