Add set of warning flags

Initally disabled keeping the old warning flags. Defining
ENABLE_WARNINGS turns them on. Flags suggested by Nordlöw in forum
thread: "Recommendation: Compile DMD with More Warnings"
Clean up compiler flag handling

All source files have a common set of build flags and those should be
put in CFLAGS, for example warnings, optimization level, and possibly
profiling flags if required.

Introduces four specific flags for the different object file sets to
keep some diversity if necessary. (This could be removed if the small
differences are insignificant).

Introduces ENABLE_DEBUG (same as DEBUG) and ENABLE_PROFILING to turn on
debugging and profile flags. ENABLE_PROFILING turns on previously
commented PROFILE and COV variables. Content is the same.