meson_options.txt: add option html_manual

doc/meson.build

 install_man(['mpd.1', 'mpd.conf.5'])
 
+if not get_option('html_manual')
+  subdir_done()
+endif
+
 sphinx = find_program('sphinx-build', required: get_option('documentation'))
 if not sphinx.found()
   subdir_done()
 endif
 
-sphinx_output = custom_target(
+if get_option('html_manual')
+  sphinx_output = custom_target(
     'HTML documentation',
     output: 'html',
     input: [
@@ -18,9 +23,9 @@ sphinx_output = custom_target(
     build_by_default: true,
     install: true,
     install_dir: join_paths(get_option('datadir'), 'doc', meson.project_name()),
-)
+  )
 
-custom_target(
+  custom_target(
     'upload',
     input: sphinx_output,
     output: 'upload',
@@ -32,6 +37,5 @@ custom_target(
       '--include=html', '--include=html/**',
       '--exclude=*',
     ],
-)
-  
-  
+  )
+endif

meson_options.txt

 option('documentation', type: 'feature', description: 'Build documentation')
+option('html_manual', type: 'boolean', value: true, description: 'Build the HTML manual')
 
 option('test', type: 'boolean', value: false, description: 'Build the unit tests and debug programs')