audio/lv2: Fix Python (3.x) build, Add missing dependencies

While lv2 itself works fine with any Python version we support, the build
fails using a Python > 3.6, since the build bundles an old version of waf.

waf upstream fixed the bug in a later version. [1]

Restrict Python to up to 3.6 for now. [2]

While here:

• Add missing dependencies identified in QA. [3]
• Pet portlint (USES order)

[1] https://gitlab.com/ita1024/waf/commit/facdc0b173d933073832c768ec1917c553cb369c

PR: 235103
Reported by: cs [3]
Submitted by: Charlie Li <ml+freebsd vishwin info> [2][3]
Approved by: Michael Beer <beerml sigma6audio de>
MFH: 2019Q2 (blanket: build fix, missing dependencies)