How to write a FreeBSD-style makefile : checking for external vars existence?

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Hello,
I was just wondering...

- -How do I go about checking for external variables, as defined by a shell?
- -How do I go about checking for variables as defined in make.conf?
- -What do variable definitions generally look like in each of these
cases, ie what's the difference between $$VAR, ${VAR}, and VAR?

The reason why I'm asking is because I'm trying to make a set of
FreeBSD makefiles and the following type of declaration doesn't work for
some odd reason..

.ifdef $$VAR
@echo define $$VAR please!
.else
#do something to build program
.endif

Make reports errors in the above case if I put anything in other
than VAR, which is confusing given my knowledge of shell scripting and
so on.
TIA,
- -Garrett
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