build: fix a warning on non-GNU distributions

Found on FreeBSD where /bin/sh descends from Almquist shell.

[[ is supported by Bash, Zsh, oksh but not by FreeBSD sh, NetBSD sh, dash, mrsh.

dash$ [[ foo = bar ]]
dash: 1: [[: not found

mrsh$ [[ foo = bar ]]
mrsh:0:0: syntax error: keyword is reserved and causes unspecified results: [[

See also https://gitlab.gnome.org/bilelmoussaoui/gtk-rust-template/-/merge_requests/52