Work around g_key_file_new() thread-safety problems

The first call to g_key_file_new() does some thread-unsafe
initialization.  Manually run that initialization from the shared library
constructor to avoid crashes or heap corruption if multiple threads
call openslide_detect_vendor() or openslide_open() in parallel.

Fixes openslide#161.

src/openslide.c

@@ -62,6 +62,9 @@ static void __attribute__((constructor)) _openslide_init(void) {
   }
   // initialize GObject
   g_type_init();
+  // work around thread-safety problems with first g_key_file_new() call
+  // https://bugzilla.gnome.org/show_bug.cgi?id=748474
+  g_get_language_names();
   // init libxml2
   xmlInitParser();
   // parse debug options