docs: add option to skip building HTML docs

Reorder meson targets to handle those all at the end, and exit early if
HTML documentation is disabled. This makes it possible to build just the
manpage, without hotdoc installed.

docs/meson.build

@@ -2,19 +2,6 @@ project('Meson documentation', version: '1.0')
 
 cur_bdir = meson.current_build_dir()
 
-# Only the script knows which files are being generated
-docs_gen = custom_target(
-    'gen_docs',
-    input: files('markdown/index.md'),
-    output: 'gen_docs.stamp',
-    command: [
-        files('../tools/regenerate_docs.py'),
-        '--output-dir', cur_bdir,
-        '--dummy-output-file', '@OUTPUT@',
-    ],
-    build_by_default: true,
-    install: false)
-
 sitemap = files('sitemap.txt')
 
 yaml_loader = get_option('unsafe_yaml') ? 'fastyaml' : 'yaml'
@@ -34,24 +21,6 @@ refman_binary = custom_target(
     ]
 )
 
-refman_md = custom_target(
-    'gen_refman_md',
-    input: refman_binary,
-    output: ['configured_sitemap.txt', 'refman_links.json'],
-    command: [
-        genrefman,
-        '-l', 'pickle',
-        '-g', 'md',
-        '-s', sitemap,
-        '-i', '@INPUT@',
-        '-o', '@OUTPUT0@',
-        '--link-defs', '@OUTPUT1@',
-        '--force-color',
-        '--no-modules',
-    ],
-)
-sitemap = refman_md[0]
-
 refman_json = custom_target(
     'gen_refman_json',
     build_by_default: true,
@@ -66,6 +35,7 @@ refman_json = custom_target(
         '--force-color',
     ],
 )
+test('validate_docs', find_program('./jsonvalidator.py'), args: [refman_json])
 
 refman_man = custom_target(
     'gen_refman_man',
@@ -83,6 +53,43 @@ refman_man = custom_target(
     ],
 )
 
+# Everything past here is HTML resources.
+if not get_option('html')
+    subdir_done()
+endif
+
+# Only the script knows which files are being generated
+docs_gen = custom_target(
+    'gen_docs',
+    input: files('markdown/index.md'),
+    output: 'gen_docs.stamp',
+    command: [
+        files('../tools/regenerate_docs.py'),
+        '--output-dir', cur_bdir,
+        '--dummy-output-file', '@OUTPUT@',
+    ],
+    build_by_default: true,
+    install: false,
+)
+
+refman_md = custom_target(
+    'gen_refman_md',
+    input: refman_binary,
+    output: ['configured_sitemap.txt', 'refman_links.json'],
+    command: [
+        genrefman,
+        '-l', 'pickle',
+        '-g', 'md',
+        '-s', sitemap,
+        '-i', '@INPUT@',
+        '-o', '@OUTPUT0@',
+        '--link-defs', '@OUTPUT1@',
+        '--force-color',
+        '--no-modules',
+    ],
+)
+sitemap = refman_md[0]
+
 genrelnotes = custom_target(
     output: ['sitemap-genrelnotes.txt'],
     build_always_stale: true,
@@ -95,8 +102,6 @@ genrelnotes = custom_target(
 )
 sitemap = genrelnotes[0]
 
-test('validate_docs', find_program('./jsonvalidator.py'), args: [refman_json])
-
 hotdoc_prog = find_program('hotdoc', version: '>=0.13.7')
 
 hotdoc = import('hotdoc')

docs/meson_options.txt

@@ -1,2 +1,4 @@
 option('unsafe_yaml', type: 'boolean', value: false,
     description: 'disable safety checks and use a faster, but less correct YAML loader')
+option('html', type: 'boolean', value: true,
+    description: 'build the hotdoc-based HTML documentation')