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

wmake/rules/linuxARM7Gcc/c

@@ -1,10 +1,9 @@
-SUFFIXES += .c
+include $(GENERAL_RULES)/Gcc/c
 
 cARCH       = -mcpu=cortex-a9
 
-cWARN       = -Wall
-
-include $(DEFAULT_RULES)/c$(WM_COMPILE_OPTION)
+# Compile option is non-mandatory, but must be non-empty
+sinclude $(DEFAULT_RULES)/c$(WM_COMPILE_OPTION)
 
 cFLAGS      = $(cARCH) $(GFLAGS) $(cWARN) $(cOPT) $(cDBUG) $(LIB_HEADER_DIRS) -fPIC