Determine library type of build from CMake variables
As g4vecgeomnav may link to non-PIC static libs, allow it to be built as static or shared based on the standard CMake variables BUILD_{STATIC,SHARED}_LIBS. The rule for choosing is:
- If neither are set, default to SHARED as now
- If one is set, that will be used
- If both are set, build a SHARED library
Fix provided by Ben Morgan.