configure.ac: create package.tar.bz2 by default

bzip2 creates archives that are better compressed than the default gzip
format. So running 'make dist' is creating a libcoap-VER.tar.bz2, you
can create a *.tar.gz by running 'make dist-gzip' if you prefer.

The created bzip2 archive is currently 27% smaller than the respective
gzip archive.

   $ ls -sh libcoap-4.1.2.tar.*
   372K libcoap-4.1.2.tar.bz2
   476K libcoap-4.1.2.tar.gz

.gitignore

@@ -14,6 +14,7 @@ configure
 debian/
 depcomp
 install-sh
+libcoap-*.tar.bz2
 libtool
 ltmain.sh
 m4/

configure.ac

@@ -15,7 +15,7 @@ m4_define([libcoap_version],[libcoap_major_version.libcoap_minor_version.libcoap
 
 AC_INIT([libcoap], [libcoap_version], [libcoap-developers@lists.sourceforge.net], [libcoap], [http://libcoap.sourceforge.net/])
 AC_PREREQ([2.59])
-AM_INIT_AUTOMAKE([1.10 -Wall no-define])
+AM_INIT_AUTOMAKE([1.10 -Wall no-define no-dist-gzip dist-bzip2])
 
 # Generate one configuration header file for building the library itself with
 # an autogenerated template. We need later a second one (include/libcoap.h)