Stop using "systemlib" as the sectionname for DSO systemlibs

This simplifies embedding for bundled extensions
being built as DSOs.

Reviewed By: @fredemmott

Differential Revision: D1246821
Unify extension macros for bundled/dynamic extensions

Now bundled extensions can be built as DSO
Merge remote-tracking branch 'origin/master' into newrelic-profiling-master
fix compile error on master/3.0 branch