fix memory leak in logging of large buffers
configure.ac: fix building without libxml2

Options --with-libxml2 and --without-libxml2 are broken. libstrophe is
always built with libxml2 support. This patch fixes the problem and
makes libstrophe be built with expat support by default.
build shared library
add libstrophe.pc file
update .gitignore
Makefile.am: preserve system defined CFLAGS
base64_decode: avoid null pointer dereference
base64_decode: handle zero-length buffers

zero-length buffer will cause problem within 'base64_decoded_len' and
with use of uninitialized 'hextet'
Merge branch 'master' of github.com:pasis/libstrophe
Merge remote-tracking branch 'rzuck/master'
xmpp_log: free memory before exit
_log_open_tag: size_t can never be less then 0

size_t is unsigned type. We should declare len as either ssize_t or int
base64_decode: shut up compiler
auth.c: remove unused declaration
thread.c: fix memory leak within mutex_destroy
ctx.c: spacing
update .gitignore
xmpp_log: do not process when handler is NULL
rename fork to libcouplet
remove expat dir