depends: Propagate only specific CLI variables to sub-makes
depends: Propagate only specific CLI variables to sub-makes

We want to supply well-known vars to ./configure scripts to do with as
they please. However, we do _not_ want to override these well-known vars
at make-time as certain build systems expect a self-mangled version of
these well-known vars.

For example, freetype and bdb will prepend `libtool --mode=compile' to
CC and CXX, which, if we override CC on the command line at make-time,
will break the build.

PIVX-Project/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX,Darknet-Crypto/Darknet,PIVX-Project/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX,Darknet-Crypto/Darknet,PIVX-Project/PIVX,Darknet-Crypto/Darknet,Darknet-Crypto/Darknet,Darknet-Crypto/Darknet,Darknet-Crypto/Darknet