build: only demand an xsltproc program when it is used

It's intrinsically used when building manpages, and also part of the
introspection-specific parts of the documentation build. There's no
particular guarantee either of those will actually be invoked during a
build, so don't unconditionally look it up.

This allows building with one fewer dependency in many cases.

docs/api/meson.build

@@ -1,6 +1,8 @@
 # SPDX-License-Identifier: LGPL-2.1-or-later
 
 if enable_introspection
+  xsltproc = find_program('xsltproc')
+
   settings = 'settings-spec'
   output = settings + '.xml'
 

man/meson.build

@@ -7,7 +7,7 @@ common_ent_file = configure_file(
 )
 
 xsltproc_options = [
-  xsltproc,
+  find_program('xsltproc'),
   '--output', '@OUTPUT@',
   '--path', meson.current_build_dir(),
   '--xinclude',

meson.build

@@ -89,7 +89,6 @@ po_dir = source_root / 'po'
 top_inc = include_directories('.')
 
 perl = find_program('perl')
-xsltproc = find_program('xsltproc')
 
 check_exports = find_program(join_paths(source_root, 'tools', 'check-exports.sh'))