--with-system-mozilla-headers -> --with-system-npapi-headers

[Stephan Bergmann]

Attention please, as 
<http://cgit.freedesktop.org/libreoffice/core/commit/?id=13ef9dcc206d30ebd4d63afb186d379dc849b36c> 
"Rename 'Mozilla headers' to 'NPAPI headers' (incl. configure option 
name)" just renamed a configure option, for better or worse.

Stephan