graphics/gtk-update-icon-cache: restore r297047 after r313633

• Invoke pkg-config(1) instead of checking manually [1]
• Convert to ECHO_MSG which can be silenced

PR: 166279
Reported by: mi (via mail on gnome@ list)
Approved by: gnome (kwm)
Submitted by: John Hein <jh-fbml@snkmail.com> [1]
Differential Revision: https://reviews.freebsd.org/D4260