squashfs-tools: Fix USE flag compressor enables
squashfs-tools: Fix USE flag compressor enables

The do_configure_* functions were always called, but never worked.

This makes sure they are only called when the corresponding USE flag
is set, and that it actually changes the Makefile.

Signed-off-by: Esben Haabendal <da90c138e4a9573086862393cde34fa33d74f6e5@haabendal.dk>

sknsean/core,sknsean/core,sknsean/core,sknsean/core,sknsean/core