build.ps1: Replaced comment header with comment-based help. Use 'Get-Help build.ps1' or 'Get-Help build.ps1 -Full'
build.ps1: Moved source URLs section next to the parameters section so that all the script internals are after the stuff that's of interest to the user.
Re-saved pango-language-sample-table.h with BOM because cl.exe throws C2001 otherwise.