Update gzdoom to 4.2.1
Update to 4.2.1.
No changelog provided at the moment of writing this, changelog should appear here later.
- Reintroduced building against FluidSynth, since GZDoom 4.2.1 supports building against FS 2.x.
Test Plan: Completed levels from DOOM 1-2.
Reviewers: Triage Team, JoshStrobl
Reviewed By: Triage Team, JoshStrobl
Differential Revision: https://dev.getsol.us/D7137