Workaround build failure on -fno-common toolchains like upstream
gcc-10. Otherwise build fails as:
ld: tcextract-extract_pcm.o:/build/transcode-1.1.7/import/extract_pcm.c:36: multiple definition of
`audio'; tcextract-extract_ac3.o:/build/transcode-1.1.7/import/extract_ac3.c:337: first defined here