meson_options.txt: add option html_manual

doc/meson.build

@@ -1,10 +1,15 @@
 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
 
+if get_option('html_manual')
   sphinx_output = custom_target(
     'HTML documentation',
     output: 'html',
@@ -33,5 +38,4 @@ custom_target(
       '--exclude=*',
     ],
   )
-  
-  
+endif

meson_options.txt

@@ -1,4 +1,5 @@
 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')