Make Guile bindings build-parallel-safe

[Ludovic Courtès]

Hello,

Commit 330995a fixes guile/src/Makefile.am to be parallel-safe, which it
wasn’t during all these years.  With recent versions of Guile 2.0, this
could lead to segfaults while running the test suite, or unbound
variable errors like:

  throw to `unbound-variable' with args (#f "Unbound variable: ~S" (set-session-certificate-type-priority!) #f)

Can you please port it to any relevant branches?

Thanks,
Ludo’.