security/pinentry-qt6: Fix configure on 13.2

Qt 6.x requires C++17 and the C++ standard is not set for the configure
script nor for the build. The base compiler on FreeBSD 13.2 defaults to
C++11, causing the configure script to fail as would the build if it
made it past the configure stage.

Reported by: pkg-fallout

(cherry picked from commit cc6cd777dae7001e3e3a8c5492a1cbbb7af5d346)