build: simplify headers management and remove recursion

There is no real reason to keep the include directory a multi-level
recursion, so instead declare everything within include (so that we
can use proper nobase_ declarations) and be it.

Please note that since we removed the sub-Makefile.am, ./configure
will not create the directory structure for us on out-of-tree builds,
so we have to make sure the directory we're generating to exists first.

Signed-off-by: Diego Elio Pettenò <flameeyes@flameeyes.eu>
diff --git a/configure.ac b/configure.ac
index 7c62f8f..728173e 100644
--- a/configure.ac
+++ b/configure.ac
@@ -10,6 +10,7 @@
 
 dnl checks for programs
 AC_PROG_MAKE_SET
+AC_PROG_MKDIR_P
 AC_PROG_CC
 AC_PROG_INSTALL
 LT_INIT
@@ -138,15 +139,6 @@
 	libosmovty.pc
 	libosmogsm.pc
 	libosmogb.pc
-	include/osmocom/Makefile
-	include/osmocom/vty/Makefile
-	include/osmocom/codec/Makefile
-	include/osmocom/crypt/Makefile
-	include/osmocom/gsm/Makefile
-	include/osmocom/gsm/protocol/Makefile
-	include/osmocom/gprs/Makefile
-	include/osmocom/gprs/protocol/Makefile
-	include/osmocom/core/Makefile
 	include/Makefile
 	src/Makefile
 	src/vty/Makefile