You receive this bug because this package does not respect my system's AR (x86_64-pc-linux-gnu-ar) and calls directly /usr/bin/ar
The possible solutions to fix this issue are:
1)Fix the buildsystem, if you can;
2)inherit toolchain-funcs and use tc-export AR
3)inherit toolchain-funcs and use emake AR="$(tc-getAR)"
From the build log:
ar rc libbrlapi.a brlapi_client.o
Fixed in app-accessibility/brltty-4.5-r2.
Thanks for the report.