Previous versions of leveldb are compiled without -fPIC which is required by bitcoin-qt. ~bitcoin-qt-9 will not build against ebuilds in tree that are <dev-libs/leveldb-1.15.0-r1.
strictly speaking, we don't support KEYWORD mixing.
How would that be an issue here? bitcoin-qt-0.9.2.1's KEYWORDS are "~amd64 ~arm ~x86", all of which are included in the KEYWORDS of leveldb-1.15.0-r1 and higher. I've seen this bug in action when I tried to compile bitcoin-qt-0.9.2.1 against leveldb-1.9.0-r6, and it failed. Compiling the same bitcoin-qt against leveldb-1.15.0-r1 was successful. (I can attach build logs if needed)
*** This bug has been marked as a duplicate of bug 504874 ***