Do not mark malloc_conf as weak for unit tests.

This is generally correct (no need for weak symbols since no jemalloc
library is involved in the link phase), and avoids linking problems
(apparently unininitialized non-NULL malloc_conf) when using cygwin with
gcc.

src/jemalloc.c

@@ -5,7 +5,11 @@
 /* Data. */
 
 /* Runtime configuration options. */
-const char	*je_malloc_conf JEMALLOC_ATTR(weak);
+const char	*je_malloc_conf
+#ifndef JEMALLOC_JET
+    JEMALLOC_ATTR(weak)
+#endif
+    ;
 bool	opt_abort =
 #ifdef JEMALLOC_DEBUG
     true