net/wmwave: fix build with -fno-common

While we are at it:

• define LICENSE
• switch to USES=localbase
• add missing include for errx()

PR: 267588
MFH: 2023Q1
Approved by: build fix blanket, maintainer timeout
Approved by: flo (mentor)
Differential Revision: https://reviews.freebsd.org/D38511

(cherry picked from commit 3ed49e6414ed7ec56e9a2d4683ea3114271bcae3)