configure.ac: Enable color-test automake option.

This will colorize the `make check` output if your term supports it.
Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>

configure.ac

@@ -22,7 +22,7 @@ AC_CANONICAL_BUILD
 AC_CANONICAL_HOST
 AC_PRESERVE_HELP_ORDER
 
-AM_INIT_AUTOMAKE(tar-ustar)
+AM_INIT_AUTOMAKE(tar-ustar color-tests)
 AM_CONFIG_HEADER(config.h)
 
 # Disable with "./configure --disable-silent-rules" or "make V=1"