Fix distcheck failure with distributed generated parallel tests.

Fix distcheck failure with distributed generated parallel tests.

* tests/Makefile.am ($(parallel_tests)): Cope with $(srcdir)
being different from the value at developer `make dist' time.
* tests/defs.in: Protect against mutiple inclusion, by using ...
($am_defs_included): ... this new variable.
($srcdir): Do not compute, but simply define to `@abs_srcdir@'
as substituted by configure.

Report, suggestions and first fix by Ralf Wildenhues, final
patch by Stefano Lattarini.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>