CONFIG: combine wmake rules for KNL architecture as a compile option

- instead of

      WM_COMPILER=GccKNL WM_COMPILE_OPTION=Opt

      -> linux64GccKNLDPInt32Opt

  now specify

      WM_COMPILER=Gcc  WM_COMPILE_OPTION=OptKNL

      -> linux64GccDPInt32OptKNL

This makes it easier (and more obvious) for adding different tweaks
without needing to generate too many files.
Eg,

    cd wmake/rules/linux64Gcc

    cp cOpt   cOptBdw
    cp c++Opt c++OptBdw

    edit these two files and then use WM_COMPILE_OPTION=OptBdw

CONFIG: provide some default c/c++ flags in General compiler rules

- can make is easier when deriving new compile options, and ensures
  that '-02' is enabled as an initial default.

wmake/rules/General/Gcc/c

@@ -0,0 +1,7 @@
+SUFFIXES += .c
+
+cc          = gcc
+
+cDBUG       =
+cOPT        = -O2
+cWARN       = -Wall

wmake/rules/General/Gcc/c++

@@ -2,6 +2,8 @@ SUFFIXES += .C .cc .cpp .cxx
 
 CC          = g++ -std=c++11
 
+c++DBUG     =
+c++OPT      = -O2
 ptFLAGS     = -DNoRepository -ftemplate-depth-100
 
 # - Standard warnings