Build using implicit rules with pattern matching

The current posix.mak declares a build-rule for each source file. Making
it somewhat hard to grasp how compiler flags affect the different files,
or at least how the *should* affect the files. The compiler flags do not
seem to be identical for all source files within each of the different
classes (DMD_OBJS, ROOT_OBJS, GLUE_OBJS and BACK_OBJS).

This change declares four implicit rules for building each of the object
classes mentioned above. It should make it eaiser to add/change/control
the compiler flags for the classes of source files. Especially adding
and changing what warnings-flags are being used.

The difference against master (d9864ef2) when using md5sum on the
resulting object files only differs because of different content in
'verstr.h' which affects 'mars.c', and because 'strtold.c' now is
compiled with flags. (Previously the explicit build rule for 'strtold.c'
did not specify any compiler flags at all).
WIP: Add pernord warnings, within ifdef