Refuse to run tex2lyx tests with in-source builds

As Günter found out running the tex2lyx tests overwrites the test references
if the build directory is identical to the source directory. Therefore the
tests would always pass, but git diff would show a non-empty diff if the tests
should have failed. Since it is better anyway to build in a separate directory
we simply do not support srcdir = builddir for the tests and abort with an
error.

src/tex2lyx/Makefile.am

@@ -67,6 +67,10 @@ TEST_RESULTS = \
 alltests: check alltests-recursive
 
 alltests-recursive: tex2lyx
+	if test "$(srcdir)" = "$(builddir)"; then \
+		echo "tex2lyx tests require a separate build directory"; \
+		exit 1; \
+	fi
 	@$(PYTHON) "$(srcdir)/test/runtests.py"; \
 	if test $$? -eq 0; then \
 		echo -e "=====================\ntex2lyx tests passed.\n====================="; \