USE_SYSTEM_GECODE=1 broken by fat dep-selector-libgecode gems

[lamont-granquist]

The USE_SYSTEM_GECODE=1 switch needs to be incorporated into this gem to switch between compiling against dep-selector-libgecode vs. the system libgecode.

If we push a fat binary for linux systems (which i tried to do with dep-selector-libgecode 1.3.0) it removes the option of setting USE_SYSTEM_GECODE=1 entirely.

I /think/ we want that environment setting to move to this gem and affect how we build the c-extension here (but its not 100% clear in my mind we couldn't do something in dep-selector-libgecode i'm just not seeing it right now).

[lamont-granquist]

Aaand i meant to cut this issue against dep-selector, anywhoo it probably needs to get reimplemented some way to make fat binary gems work.