REORG: ebtree: clean up remains of the ebtree/ directory

The only leftovers were the unused compiler.h file and the LICENSE file
which is already mentioned in each and every ebtree file header.

A few build paths were updated in the contrib/ directory not to mention
this directory anymore, and all its occurrences were dropped from the
main makefile. From now on no other include path but include/ will be
needed anymore to build any file.
diff --git a/contrib/hpack/gen-enc.c b/contrib/hpack/gen-enc.c
index 44b643c..dfe4705 100644
--- a/contrib/hpack/gen-enc.c
+++ b/contrib/hpack/gen-enc.c
@@ -5,7 +5,7 @@
  * table is built to access the first entry of each length.
  *
  * Build like this :
- *    gcc -I../../include -I../../ebtree -o gen-enc gen-enc.c
+ *    gcc -I../../include -o gen-enc gen-enc.c
  */
 #define HPACK_STANDALONE