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