tidyp: Fix for Linuxbrew
tidyp: Fix for Linuxbrew

/usr/bin/xsltproc might not be available on some systems.
Depend on libxslt except on macOS and use its xsltproc.

Closes Linuxbrew/homebrew-core#1570.

Signed-off-by: Bob W. Hogg <c772a964fd55352a3510e5d535dd9ccc9ac30168@linux.com>

rwhogg/homebrew-core,LinuxbrewTestBot/homebrew-core,maxim-belkin/homebrew-core,Linuxbrew/homebrew-core,Linuxbrew/homebrew-core,rwhogg/homebrew-core,LinuxbrewTestBot/homebrew-core,maxim-belkin/homebrew-core