So we need to use the expat subdir too in
--with-expat-includes. Apparently until now it did not find expat.h,
so it fell back to libxml2 instead then, which was sometimes found OK,
sometimes (on tinderboxes) not. (It even went looking in the *build*
platform /usr/include, eek!)

Change-Id: If0595b810d531b5aa7110f375d4d0dfb0b01617b