lib: Define GUESTFS_GCC_VERSION in the internal header.

Note that you have to use this as follows:

#if defined(__GNUC__) && GUESTFS_GCC_VERSION >= x0y0z /* gcc >= x.y.z */

since GUESTFS_GCC_VERSION is not defined unless it's GCC.
daemon: Force disable -Wsuggest-attribute=format warning around asprintf_nowarn, for GCC 4.8.
lib: Add format attribute for GCC 4.8 in two places.