Make m4 only call macros surrounded by brackets.

[Zack Piper]

Hello,

I'm currently trying to figure out how to make m4 only call macros within 
certain brackets, for example:

```
define(`test', '2')
test
->test

<test>
->2
```

It would be even better if this was possible.

```
define(`test', '2')
<test>
-><test>

<#test>
->2
```

Above is exactly what the `-H` flag does on GPP (Generic Preprocessor) 
(http://files.nothingisreal.com/software/gpp/gpp.html).

Any help would be great, thanks!
-- 
I have nothing fancy or intelligent to put here.

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