Build using implicit rules with pattern matching

This change declares four implicit rules for building each of the set of
objects (DMD_OBJS, ROOT_OBJS, GLUE_OBJS and BACK_OBJS). It should make
it easier to add/change/control the compiler flags for the different set
of source files. Especially adding and changing warnings-flags being
used.

The difference against master (db2291f) on the object files only differs
(using md5sum) because of different content in 'verstr.h' which affects
'mars.c', and because 'strtold.c' is now compiled with flags and is
affected by optimization.

(I realize this is a rather big change but hopefully it will de-complicate the posix.mak a bit :)