how to override platform supplied tool with inappropriate version?

[Marko Schütz Schmuck <MarkoSchuetz%web.de@localhost>]

Specifically, I want to build (unprivileged) devel/autoconf, which has 

USE_TOOLS+=   gm4:run perl:run

in the Makefile. The platform is Linux, but the version of /usr/bin/m4
is too old: 1.4.4.

So, what is the "right" way to override TOOLS_PLATFORM.gm4
conditionally? I briefly tried something like

M4_VERSION!=   ${M4} --version | ${AWK} ...

but M4 is not defined at that point yet...

Best regards,

Marko

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