meson: introduce a "docs" alias to compile documentation only

Meson does not currently provide a target to compile documentation,
only. Instead, users needs to compile the whole project, which may be
way more than they really intend to do.

Introduce a new "docs" alias to plug this gap. This alias can be invoked
e.g. with `meson compile docs`.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/howto/meson.build

@@ -29,7 +29,7 @@ howto_index = custom_target(
   output: 'howto-index.adoc',
 )
 
-custom_target(
+doc_targets += custom_target(
   command: asciidoc_html_options,
   input: howto_index,
   output: 'howto-index.html',
@@ -51,7 +51,7 @@ foreach howto : howto_sources
     capture: true,
   )
 
-  custom_target(
+  doc_targets += custom_target(
     command: asciidoc_html_options,
     input: howto_stripped,
     output: fs.stem(howto_stripped.full_path()) + '.html',

Documentation/meson.build

@@ -375,7 +375,7 @@ foreach manpage, category : manpages
       output: fs.stem(manpage) + '.xml',
     )
 
-    custom_target(
+    doc_targets += custom_target(
       command: [
         xmlto,
         '-m', '@INPUT0@',
@@ -398,7 +398,7 @@ foreach manpage, category : manpages
   endif
 
   if get_option('docs').contains('html')
-    custom_target(
+    doc_targets += custom_target(
       command: asciidoc_common_options + [
         '--backend=' + asciidoc_html,
         '--doctype=manpage',
@@ -450,7 +450,7 @@ if get_option('docs').contains('html')
     depends: documentation_deps,
   )
 
-  custom_target(
+  doc_targets += custom_target(
     command: [
       xsltproc,
       '--xinclude',
@@ -479,7 +479,7 @@ if get_option('docs').contains('html')
   ]
 
   foreach article : articles
-    custom_target(
+    doc_targets += custom_target(
       command: asciidoc_common_options + [
         '--backend=' + asciidoc_html,
         '--out-file=@OUTPUT@',

Documentation/technical/meson.build

@@ -46,7 +46,7 @@ api_index = custom_target(
   output: 'api-index.adoc',
 )
 
-custom_target(
+doc_targets += custom_target(
   command: asciidoc_html_options,
   input: api_index,
   output: 'api-index.html',
@@ -56,7 +56,7 @@ custom_target(
 )
 
 foreach article : api_docs + articles
-  custom_target(
+  doc_targets += custom_target(
     command: asciidoc_html_options,
     input: article,
     output: fs.stem(article) + '.html',