Re-factor into separate compression module

Introduce a `conn_interface` to simplify the decision logic which API
we must call.

This also fixes some bugs of the previous commit.

Signed-off-by: Steffen Jaeckel <jaeckel-floss@eyet-services.de>

Makefile.am

@@ -85,6 +85,12 @@ if NEED_SNPRINTF
 libstrophe_la_SOURCES += src/snprintf.c
 endif
 
+if DISABLE_COMPRESSION
+libstrophe_la_SOURCES += src/compression_dummy.c
+else
+libstrophe_la_SOURCES += src/compression.c
+endif
+
 if DISABLE_TLS
 libstrophe_la_SOURCES += src/tls_dummy.c
 else