accept a DESTDIR environment variable for libopencm3

differing between ${PREFIX} and ${DESTDIR}/${PREFIX} is important for
creating a debian package, where all hardwired paths have to be set to
${PREFIX}/something, while the actual installation will go to
${DESTDIR}/${PREFIX}/something.

as opposed to the seemingly abandoned approach at DESTDIR from destdir_support branch, this patch wants to work just for already set environment variables, and not for argument passing. as all build systems but libopencm3's already respect a DESTDIR variable, barely anything needs changing.