libglnx: throw in custom basename implementation

libglnx tries to avoid using a specific implementation of basename by
doing #undef basename.  This trick doesn't work with glibc anymore
apparently.

But basename is like 3 lines of code to write, so just write it.

diff --git a/libglnx/glnx-fdio.h b/libglnx/glnx-fdio.h
--- a/libglnx/glnx-fdio.h	2017-11-08 13:41:49.734931484 -0500
+++ b/libglnx/glnx-fdio.h	2017-11-08 13:45:33.330758205 -0500
@@ -43,7 +43,14 @@ G_BEGIN_DECLS
 static inline
 const char *glnx_basename (const char *path)
 {
-  return (basename) (path);
+  gchar *base;
+
+  base = strrchr (path, G_DIR_SEPARATOR);
+
+  if (base)
+    return base + 1;
+
+  return path;
 }
 
 gboolean
diff --git a/libostree-2017.2/libglnx/glnx-fdio.h b/libostree-2017.2/libglnx/glnx-fdio.h
--- a/libostree-2017.2/libglnx/glnx-fdio.h	2017-11-08 13:53:45.655413948 -0500
+++ b/libostree-2017.2/libglnx/glnx-fdio.h	2017-11-08 13:53:57.543285889 -0500
@@ -43,7 +43,14 @@ G_BEGIN_DECLS
 static inline
 const char *glnx_basename (const char *path)
 {
-  return (basename) (path);
+  gchar *base;
+
+  base = strrchr (path, G_DIR_SEPARATOR);
+
+  if (base)
+    return base + 1;
+
+  return path;
 }
 
 gboolean