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/Clang/c

@@ -1,10 +1,16 @@
-SUFFIXES += .c
+#------------------------------------------------------------------------------
+# clang compiler
+#------------------------------------------------------------------------------
+SUFFIXES   += .c
 
-cc          = clang$(COMPILER_VERSION)
+cc         := clang$(COMPILER_VERSION)
 
-cARCH       =
-cDBUG       =
-cOPT        = -O2
-cWARN       = -Wall
+cARCH      :=
+cDBUG      :=
+cOPT       := -O2
+cWARN      := -Wall
+
+# Tuning
+sinclude $(GENERAL_RULES)/common/c$(WM_COMPILE_OPTION)
 
 #------------------------------------------------------------------------------