This was found originally when trying to use automake-1.13 (including
attempt with 1.13.3) for gtk-engines-2.20.2, but I've tried to produce
minimum Makefile.am to reproduce the problem (attached).

 When variable TESTS gets assigned result from patsubst -function,
running make for resulting Makefile errors with:
Makefile:592: *** unterminated variable reference.  Stop.


 Attached testcase includes minimum configure.ac and Makefile.am and
separate ./run.sh script to produce rest of the files and to run make.


 - ML