Add option to use existing cross-compiler in a canadian build

[QBos07]

Using an already existing nativ cross-compiler could speed a canadian build up dramaticaly.
Use existing x x y-toolchain to build new x z y-toolchain. (build host taget)
espacially in combination with #2061 useful

[cpackham]

You always need to have an existing toolchain for the canadian builds. CT_HOST is the variable that tells you target triplet for that toolchain and we expect to be able to find it on $PATH. Whether you've just build it or have it already existing on you system ct-ng doesn't particularly care.

[QBos07]

You mean the x x z-toochain. I mean if we can add an option the go away from building 2 gcc's where one is for build and one is for host is cases were the one for the build is already existing.