Generally, that would be in this case, but I also have added two extensions to the PHP BUILD, which is basically ffmpeg and magick PECL extensions, which is to be compiled statically into the PHP itself.
So when I also add the LSAPI 6.0 and run the buildconf, that when it barfs for it. I am still trying few ideas to try to get around that issue, but I think I found the answer by your comment, which is to build it first successfully with the extensions added by phpbuild, and then patch the sapi directory with new 6.0 and build it again from there to get it compiled.
I will update later tomorrow when I get back to it.