CONFIG: add support for WM_COMPILE_OPTION='Dbg'

- uses '-g -DFULLDEBUG' (like Debug), but with -O3 (like Opt).

  This adds in debug symbols and FULLDEBUG code segments (good for
  code development) but retains -O3 optimizations and code paths and
  avoids the much slower -O0 associated with 'Debug'.

- add in central wmake/General/common/{c,c++}XXX tuning,
  which helps reduce the number of nearly identical files

ENH: add support for wmake -debug-Og

wmake/rules/General/general

@@ -34,7 +34,7 @@ endif
 
 # Default compilation is 'Opt' - never permit an empty value
 ifeq (,$(WM_COMPILE_OPTION))
-    WM_COMPILE_OPTION = Opt
+    WM_COMPILE_OPTION := Opt
 endif
 
 # Older macro for (shell) branching