Check for local libs prior to building duplicates

[barracuda156]

The build system straightaway builds its own zlib etc., this is suboptimal: instead, configure should check for existing libs and build its own only when there are none found (or versions are too old, if there is a particular requirement).

[zhengxwen]

Thanks for the suggestion.

[barracuda156]

P. S. I guess, I rather say “configure could check”, since you obviously know better what it should do; if something relies on an API specific to a particular version of a dependency, and thus requires a static lib, then my suggestion will not apply.