FindDocBookXSL.cmake: Add

Taken from
<https://github.com/KDE/kdoctools/blob/master/cmake/FindDocBookXSL.cmake>

Signed-off-by: Simon McVittie <smcv@collabora.com>
Resolves: #237
12 jobs for master in 17 minutes and 4 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Build
passed #46669
build:cmake

00:06:26

passed #46666
build:debug

00:09:10

manual #46671
allowed to fail manual
build:i686-w64-mingw32-cmake
passed #46670
build:i686-w64-mingw32-debug

00:08:44

manual #46674
allowed to fail manual
build:jessie
manual #46668
allowed to fail manual
build:legacy
passed #46665
build:production

00:07:43

manual #46667
allowed to fail manual
build:reduced
manual #46676
allowed to fail manual
build:trusty
passed #46672
build:x86_64-w64-mingw32

00:08:12

passed #46673
build:x86_64-w64-mingw32-cmake-debug

00:07:50

manual #46675
allowed to fail manual
build:xenial