Use mkdir macro also in case if mingw32.

This fixes bug 676511.
Use $libdir for LIBXSLT_DEFAULT_PLUGINS_PATH

Fixes bug #736204.

https://bugzilla.gnome.org/show_bug.cgi?id=736204
Check return value of exsltFuncNewFunctionData

Fixes bug #735957.

https://bugzilla.gnome.org/show_bug.cgi?id=735957

Thanks to Prabhat Kanth for the report.