audio/artyfx-lv2: fix build with GCC-based architectures
The fix for clang in files/patch-src_avtk_avtk_picojson.hxx breaks GCC. Have two
versions for two compilers.
Fix altivec in src/avtk/libs/pffft.cxx and add -maltivec to CXXFLAGS on
powerpc64.
Also use new compilers.
PR: 239518
Approved by: yuri (maintainer), tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D21141