m4: Make easier to integrate in other projects

Add a common.m4 file to be included directly in other project.

This include will include all needed checks to use spice-common.

Just include directly this file in your configure.ac.

This will define SPICE_COMMON_CFLAGS and SPICE_COMMON_DIR (for linking .la files) which are needed to use spice-common.

Signed-off-by: Frediano Ziglio <fziglio@redhat.com>

This is the related change for spice-server: fziglio/spice@1924a631