cmake variable sc_external used before defined

[Pierre Kestener]

Description

In top-level CMakeLists.txt variable sc_external is used at line 33 (https://github.com/cburstedde/p4est/blob/master/CMakeLists.txt#L33), while it is defined inside cmake/sc.cmake that is not sourced yet.

Proposed solution

As sc_external behave like an option, just make it actually a cmake option and move its definition inside cmake/options.cmake

[Carsten Burstedde]

Thanks; this appears resolved! :)