Try to avoid __imp__xmlFree link trouble on msys

* include/libxml/xmlexports.h: when compiling with mingw/MSYS or linking
  to an precompiled library this _imp__xmlFree missing at runtime is a
  common problem. Igor and various people faced it and this seems the
  minimal fix for it, should resolve 590302 and 561340
diff --git a/include/libxml/xmlexports.h b/include/libxml/xmlexports.h
index 29a6f54..b90659f 100644
--- a/include/libxml/xmlexports.h
+++ b/include/libxml/xmlexports.h
@@ -108,7 +108,12 @@
   #undef XMLPUBVAR
   #undef XMLCALL
   #undef XMLCDECL
-  #if defined(IN_LIBXML) && !defined(LIBXML_STATIC)
+  /*
+   * if defined(IN_LIBXML) this raises problems on mingw with msys
+   * _imp__xmlFree listed as missing. Try to workaround the problem
+   * by also making that declaration when compiling client code.
+   */
+  #if !defined(LIBXML_STATIC)
     #define XMLPUBFUN __declspec(dllexport)
     #define XMLPUBVAR __declspec(dllexport)
   #else