Any particular reason why this targets PHP-7.2, and not PHP-7.1 or master?

IMHO, when available, we should rely on pkg-config output

I wrote it for 7.2.7 and have not checked for compatibility with 7.1. As for pkg-config, if we trust it, then it still prints /usr/include, which won't work for the check, but the /usr/include/x86_64-linux-gnu directory is still part of the global includes on the platform. The configure script will fail, even though the build can succeed.

which won't work for the check

when using pkg-config output, you DON'T have to check if the file exists, simply rely on its output (see other ext. which already use it)

Comment on behalf of petk at php.net:

Updating labels

I have implemented the pkg-config migration of ext/gd in #3739, which should resolve this issue in the most general way.