This merge introduces the evaluation of optional multiline defines
'forLinux', 'forDarwin' and 'forWindows', which can be used as a flexible
replacement for system-specific variables as in Pd-extended's Makefile
template. Example code is added to Makefile.pdlibbuilder's built-in
documentation.
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.
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.
