lang/php73: Use pkg-config to detect freetype

In PHP 7.4 and above it is already in use.
Also ignore PHP 7.2 because it is scheduled for removal.

PR: 252115
Submitted by: <daniel.engberg.lists@pyret.net>
Obtained from: https://aur.archlinux.org/cgit/aur.git/tree/freetype.patch?h=php73