Mk/bsd.sanity.mk: Properly escape USE_QT4_ALT

% make
/!\ qtractor-0.9.9: Makefile errors /!\

/bin/sh: Syntax error: "(" unexpected

• Error code 2

PR: 239814
Reported by: Radim Kolar <hsn@sendmail.cz>
Approved by: portmgr (rene)
Differential Revision: https://reviews.freebsd.org/D21474