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/linux64Mingw/c

@@ -1,12 +1,14 @@
 #------------------------------------------------------------------------------
+# mingw is gcc
+#------------------------------------------------------------------------------
 include $(GENERAL_RULES)/Gcc/c
 
-cARCH       = -m64
+cc         := x86_64-w64-mingw32-gcc
+cARCH      := -m64
 
-cc          = x86_64-w64-mingw32-gcc
-
-# Compile option is non-mandatory, but must be non-empty
-sinclude $(RULES)/c$(WM_COMPILE_OPTION)
+ifneq (,$(WM_COMPILE_OPTION))
+    sinclude $(DEFAULT_RULES)/c$(WM_COMPILE_OPTION)
+endif
 
 cFLAGS      = \
     $(cARCH) $(GFLAGS) $(cWARN) $(cOPT) $(cDBUG) \