Do not mark malloc_conf as weak on Windows.

This works around malloc_conf not being properly initialized by at least
the cygwin toolchain.  Prior build system changes to use
-Wl,--[no-]whole-archive may be necessary for malloc_conf resolution to
work properly as a non-weak symbol (not tested).

src/jemalloc.c

@@ -6,7 +6,7 @@
 
 /* Runtime configuration options. */
 const char	*je_malloc_conf
-#ifndef JEMALLOC_JET
+#ifndef _WIN32
     JEMALLOC_ATTR(weak)
 #endif
     ;