audio/gmtp: Update to 1.3.11

Add temporary workaround to fix linking with Clang >= 11 and GCC >= 10

Install license with the license framework instead of listing in PORTDOCS and
convert to UNIX style newline

PR: 244712
Submitted by: <lgfbsd@be-well.ilk.org> (maintainer)
MFH: 2021Q1 (build fix)