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/decode.c b/contrib/hpack/decode.c
index 82f28c7..ce3b484 100644
--- a/contrib/hpack/decode.c
+++ b/contrib/hpack/decode.c
@@ -7,7 +7,7 @@
  * The DHT size may optionally be changed in argv[1].
  *
  * Build like this :
- *    gcc -I../../include -I../../ebtree -O0 -g -fno-strict-aliasing -fwrapv \
+ *    gcc -I../../include -O0 -g -fno-strict-aliasing -fwrapv \
  *        -o decode decode.c
  */