build: Change PKGCONF to PKG_CONFIG
build: Change PKGCONF to PKG_CONFIG

This is standard with autotools.

unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso