Problem: logic of configure tests for dependency is too complex
Problem: logic of configure tests for dependency is too complex

Solutions:
* when testing for a dependency, unify the success (or error) check using a common result variable (or abort early in fatal situations).
* process the unified result as a "case" block (dependency was looked for and found via pkgconfig, or via compile-tests, or not found/not searched with either of the methods)
* also report if "test" is not set more reasonably (now it was a no-op, and with a GSL fatal abort())

Context: this simplification improvese readability of the code, and makes possible a subsequent PR to select one of several suitable variants for the single logical dependency - which would repeat these blocks a few times, and rather do so sequentially than as nested if-elses.

paddor/zproject,taotetek/zproject,paddor/zproject,zeromq/zproject,zeromq/zproject,zeromq/zproject,paddor/zproject,taotetek/zproject,taotetek/zproject