ENH: distinguish OpenFOAM version for user-coding (fixes #195)

The pre-processor macro 'OPENFOAM_PLUS' is defined with a numerical
value equal to the currently compatible version number.

This can be used judiciously within user coding to help with minor
differences between OpenFOAM versions. For example,

    #ifdef OPENFOAM_PLUS
        #if (OPENFOAM_PLUS >= 1612)
        ...
        #endif
    #endif

or simply

    #if (OPENFOAM_PLUS >= 1612)
    ...
    #endif

wmake/rules/General/general

 #-------------------------------*- makefile -*---------------------------------
+WM_VERSION = OPENFOAM_PLUS=1606
 
 AR         = ar
 ARFLAGS    = cr
@@ -6,7 +7,7 @@ RANLIB     = ranlib
 CPP        = cpp
 LD         = ld
 
-GFLAGS     = -D$(WM_ARCH) -DWM_ARCH_OPTION=$(WM_ARCH_OPTION) \
+GFLAGS     = -D$(WM_VERSION) -D$(WM_ARCH) -DWM_ARCH_OPTION=$(WM_ARCH_OPTION) \
              -DWM_$(WM_PRECISION_OPTION) -DWM_LABEL_SIZE=$(WM_LABEL_SIZE)
 GINC       =
 GLIBS      = -lm