Makefile.am is GNU Make specific
The shell command
$(SED) -e 's,[@]PYTHON[@],$(PYTHON),' < $< > $@
expands to
/usr/bin/sed -e 's,[@]PYTHON[@],/usr/local/bin/python,' < > tester.py
when using BSD make.
The simple solution would be to name the single source in the shell command, like so
tester_py_SOURCES = src/test/tester.py.in
tester.py: $(tester_py_SOURCES) Makefile
(SED) -e 's,[@]PYTHON[@],
(PYTHON),' < $(tester_py_SOURCES) > $@
chmod +x $@
This isn't very nice though since it'll break when more files are added to tester_py_SOURCES.