Makefile.legacy: avoid make warnings for generic builds

Compiling: DES benchmark (code version #1)
make[1]: Entering directory `/home/fd/git/JtR/src'
make[1]: warning: jobserver unavailable: using -j1.  Add `+' to parent make rule.

Instead of adding '+' to the "parent" make rule I passed -j1 to ./best.sh
Makefile.legacy: make sure we really use the intended compiler for aes subdir