buildsys: cache build flags in vars

Running `make clean` atm takes like 20 seconds because every subdir
re-evaluates all the toolchain flags.  Add some helpers to automate
the process of setting up variables to cache the result of tests,
as well as the checking of an already set flag.

Now `make clean` takes like 2 seconds on my system.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>

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