Add a zcash-gtest binary to our build with a single tautological test.

src/Makefile.gtest.include

@@ -0,0 +1,13 @@
+bin_PROGRAMS += zcash-gtest
+
+# tool for generating our public parameters
+zcash_gtest_SOURCES = \
+	gtest/main.cpp \
+	gtest/test_tautology.cpp
+
+zcash_gtest_LDADD = \
+  -lgtest \
+  $(LIBBITCOIN_UTIL)
+
+zcash-gtest_check: zcash-gtest FORCE
+	./zcash-gtest