Using Custom Variables with Custom Makefile Make Command?

[minipc]

Question:
What, if any, custom variables can be used in a project's 'Build options - "Make" commands' when it employs a Custom Makefile?

Background:
I am trying to use C::B to work on a project that was originally developed in a MinGW/MSYS environment.  I've created a blank project, added the sources including the original Makefile, pointed the Project settings to it and marked it as a custom makefile.  I've changed the Build targets to match those of the Makefile and adjusted the build options "Make" commands accordingly.  The Makefile includes statements of the following form in order to switch the target architecture of the build from POSIX to Win32 by defining WIN32=1

Code

# Win32 options
ifeq ($(WIN32), 1)
	CFLAGS	+=	-mwindows -mms-bitfields
	LDFLAGS	+=	-lopengl32 -lglu32
else
	LDFLAGS += -lGL -lGLU
endif

The problem:
So far the only way I can get C::B to compile is to add the WIN32 definition to the 'Build project/target - "Make" command' directly, thus

Code

$make -f $makefile $target WIN32=1

The question (re-formed):
Is this the C::B way, or should I be entering the WIN32=1 definition in some other build option setting and adding $something to the Make command?

I've read the manual, checked out the FAQ and scoured the forums, without success.  So if I've missed something, or if this is a real dumb question, sorry.  But if someone with more C::B experience than me could help, I'd be beholding.  And if any C::B developers are reading this, then kudos to you guys and gals for your efforts:)