boost: use backtick instead of $(shell ...) make function

Also remove the variables TARGET_CC_VERSION and HOST_CC_VERSION which
lack the BOOST_ prefix and are used only once.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>