Document why we build memwipe that way.

src/test/include.am

@@ -170,8 +170,11 @@ src_test_test_slow_LDADD = $(src_test_test_LDADD)
 src_test_test_slow_LDFLAGS = $(src_test_test_LDFLAGS)
 
 src_test_test_memwipe_CPPFLAGS = $(src_test_test_CPPFLAGS)
+# Don't use bugtrap cflags here: memwipe tests require memory violations.
 src_test_test_memwipe_CFLAGS = $(TEST_CFLAGS)
 src_test_test_memwipe_LDADD = $(src_test_test_LDADD)
+# The LDFLAGS need to include the bugtrap cflags, or else we won't link
+# successfully with the libraries built with them.
 src_test_test_memwipe_LDFLAGS = $(src_test_test_LDFLAGS) @CFLAGS_BUGTRAP@
 
 src_test_bench_LDFLAGS = @TOR_LDFLAGS_zlib@ @TOR_LDFLAGS_openssl@ \