[Chapel Merge] Upgraded the third-party FLTK version from 1.3.5 t

Branch: refs/heads/main
Revision: 5f52b29
Author: jeremiah-corrado
Link: upgraded fltk version from 1.3.5 to 1.3.8 by jeremiah-corrado · Pull Request #20446 · chapel-lang/chapel · GitHub
Log Message:

Merge pull request #20446 from jeremiah-corrado/update-fltk-version

Upgraded the third-party FLTK version from 1.3.5 to 1.3.8

The chplvis tool is now building on M1 mac.

[reviewed by @mppf]

Modified Files:
A third-party/fltk/fltk-1.3.8-source.tar.gz

R third-party/fltk/fltk-1.3.5-source.tar.gz
M third-party/fltk/Makefile.include

Compare: https://github.com/chapel-lang/chapel/compare/ddf255ac108f...5f52b29d0778