Add unsupported --with-system-leveldb configure flag #5085

This enables downstream/packagers to use mainline code completely unpatched, provided they take the extra care on LevelDB required for consensus.

I'll leave it up to the others to decide if this is really wanted, but if so, please use the usual macros for checking libs/headers so that cross works as intended. Also, if they're not found when the switch was given, configure should exit with an error.

@theuni I'm not sure what you're suggesting for cross - the usual macros would detect missing libs, but there's no pkg-config information for LevelDB, so it's just going to set the variables the same way. Shouldn't be any problem with cross-compiling regardless. Anyhow, added that for the errors.

@luke-jr it checks for the existence and usability of the libs for the current host. If compiling for arm and only an x86 lib is encountered, configure will exit.

I think this is a bad idea. This is the kind of thing dostream packagers will eat up and use very gladly...without reading "DANGEROUS; NOT SUPPORTED" or misinterpreting it to mean something else.

Of course thats not to say they wont do it anyway, but making it easier...?

I tend to agree with @TheBlueMatt. I also remember we had this discussion before. Back then we put the bar at "easily patchable". Making it a configure option makes it too easy, so NACK.

(The reason that we include leveldb is that it gives us control over the (consensus-critical) database, and allows patching problems that affect us before upstream if that is necessary. That hasn't changed since then)