lib/sod.h: Make the `_vt' member of `sod_instance' be pointer-to-const.

[sod] / sod.pc.in

prefix=@prefix@
exec_prefix=@exec_prefix@
libdir=@libdir@
includedir=@includedir@

Name: sod
Description: Sensible object design, runtime library
Version: @VERSION@
Libs: -L${libdir} -lsod
Cflags: -I${includedir}