CONFIG: relax requirement on C compile option

- with 1906, OpenFOAM is purely C++ code and the last toolchain bits C
  code have been avoided. Thus relax the wmake C rule for including
  WM_COMPILE_OPTION. This makes it much easier to add different
  compiler options with fewer files.

  For example, for Broadwell-specific options:

      cd wmake/rules/linux64Gcc
      cp c++Opt c++OptBdw

      edit this file and then use WM_COMPILE_OPTION=OptBdw

- ensure that WM_COMPILE_OPTION is always non-empty

etc/config.sh/settings

@@ -20,7 +20,7 @@
 #
 #------------------------------------------------------------------------------
 export WM_ARCH="$(uname -s)"                # System name
-## : ${WM_OSTYPE:=POSIX}; export WM_OSTYPE     # System type (POSIX is default)
+: ${WM_COMPILE_OPTION:=Opt}; export WM_COMPILE_OPTION  # Default: Optimize
 
 archOption="${WM_ARCH_OPTION:-64}"
 unset WM_ARCH_OPTION        # Compiling 32-bit on 64-bit system