[scrapers] Support XSLT transforms (XML->XML) in the scrapers
cosmetic: indenting
- don't build xsltproc - it pulls in a lot of dependencies which are not available for rbpi (and so linker would try to link in x86 libs found in the system - which fails for arm linkage)
- force our libxml2 - fixes compilation on rbpi
Merge pull request #3758 from jmarshallnz/xslt_scraper

XSLT support in scrapers