Add tokenizers_srcdir_relative build tag to allow static library path

[RJKeevil]

Add tokenizers_srcdir_relative build tag to allow library to be placed either in the module source directory, or in the static /usr/lib/ path.

Due to the suggested naming of the variable, I have therefore made the relative path option non-default. If you would prefer the current behaviour to be unchanged, should we instead make the tag tokenizers_srcdir_static or something?

[jmoney]

I think https://github.com/daulet/tokenizers/pull/18 solves this issue too.

[daulet]

I believe https://github.com/daulet/tokenizers/pull/18 solves this more generically.