Only build tbbmalloc[_proxy] if requested #3661

Job	Run time
mix/fun tests 1	1h 55m 44s
prim and rev tests	4h 0m 41s
fwd tests and non-fun mix tests	1h 11m 24s
mix/fun tests 2	3h 15m 43s
	10h 23m 32s

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