make -C libpq check fails obscurely if tap tests are disabled

[Justin Pryzby]

make -C ./src/interfaces/libpq check
PATH=... && @echo "TAP tests not enabled. Try configuring with 
--enable-tap-tests"
/bin/sh: 1: @echo: not found

make is telling the shell to run "@echo" , rather than running "echo" silently.

Since:

commit 6b04abdfc5e0653542ac5d586e639185a8c61a39
Author: Andres Freund <and...@anarazel.de>
Date:   Sat Feb 26 16:51:47 2022 -0800

    Run tap tests in src/interfaces/libpq.

-- 
Justin