Targets pre and post can be defined in the including (library) makefile.
They are added to provide extra flexibility. Target all will
build pre, $(executable) and post in deterministic order. Built-in
documentation is modified to reflect the new situation.
This commit reorganizes the accumulation of build flags in such a way that
non-obligatory build flags can be overriden or added by specifying
CPPFLAGS, CFLAGS and LDFLAGS as arguments from command line or environment.
lib.version is only expanded if the file named by $(metafile) exists, but then it uses a hardcoded filename to do the actual expansion.
this patch uses $(metafile) to do the expansion instead.
