The syntax error might be caused by PKG_CHECK_MODULES being not defined,
which might be caused by an phpize of an older version. Did you use
phpize of that Git checkout?
on the build above, no.
that was a 'bootstrap' build ... using disto-installed, older php -- and its 'phpize'.
but, i also tried a 2-stage build ...
rm'ing the distro-install, building the git co, withOUT sodium ext, THEN building the sodium ext using the just-built, git co's phpize
unfortunately, same issue with failed sodium ext build.