After a day surfing the net, I humbly admit defeat. There seem to be no implementation of sysv except the one from libffi for iOS. Due my scarce comprehension of assembly, I cannot say what is done there and wouldn't be able to replicate it, anyway.
I do lack specific knowledge of what actually sysv does and how can implemented in a system not providing it by default. To solve it in an elegant way there should be someone either experienced in system programming and/or someone able to disclose some of Apple's internal stuff. I don't happen to know any of this kind of person.
As a last attempt, I'll try to make configure NOT find headers for SysV and see what happens.
I'd be interested if any other member of ininjas can provide better insight of the problem and/or figured something more out.
As a last note, I think that APR rely on SysV, which would explain why it's not possible to build a working one.