build: add LevelDB CMake detection

[Kangie]

While updating a Gentoo ebuild to use the new CMake builds I encountered some nasty red text and an inability to proceed with the build. This PR adds the missing Findleveldb.cmake and makes it useful.

[Kangie]

I'm happy to drop this based on advice:

Gridcoin-Research/src/CMakeLists.txt

Line 34 in 20629e2

set(LIBLEVELDB leveldb)

However it caused issues testing against an older version of LevelDB on my system (1.20) and doesn't appear to cause any build issues (i.e. tests still pass, etc).

[div72]

@Kangie System LevelDB detection seem to work with the *.cmake provided by dev-libs/leveldb package on Gentoo, why is this patch necessary?

[Kangie]

@Kangie System LevelDB detection seem to work with the *.cmake provided by dev-libs/leveldb package on Gentoo, why is this patch necessary?

I couldn't make it work when building via portage (or iirc locally). I can retest that over the next week.

[CyberTailor]

LevelDB detection works without this patch via system-installed *.cmake files

[div72]

LevelDB detection works without this patch via system-installed *.cmake files

Closing then. @Kangie Please re-open if you can reproduce.