audio/nekobi-lv2: fix build with GCC-based architectures

This port requires C++11-compatible compiler to build on GCC architectures.

Additionally, remove -mtune=generic -msse -msse2 on GCC architectures.

PR: D20112
Approved by: yuri (maintainer), mat (mentor)
Differential Revision: https://reviews.freebsd.org/D20112