meson: bail out earlier in docs subdir if docs are disabled

The gst_dep.get_variable('libexecdir') may fail in some scenarios
(e.g. building a module alone inside an uninstalled devenv) and
it shouldn't really be reached in the first place if docs are
disabled via options.

Also to avoid confusing meson messages when cross-compiling or
doing a static build.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/7818>

subprojects/gst-devtools/docs/meson.build

@@ -1,5 +1,9 @@
 build_hotdoc = false
 
+if get_option('doc').disabled()
+  subdir_done()
+endif
+
 if meson.is_cross_build()
     if get_option('doc').enabled()
         error('Documentation enabled but building the doc while cross building is not supported yet.')

subprojects/gst-editing-services/docs/meson.build

@@ -1,5 +1,9 @@
 build_hotdoc = false
 
+if get_option('doc').disabled()
+  subdir_done()
+endif
+
 if meson.is_cross_build()
     if get_option('doc').enabled()
         error('Documentation enabled but building the doc while cross building is not supported yet.')

subprojects/gst-libav/docs/meson.build

@@ -1,5 +1,9 @@
 build_hotdoc = false
 
+if get_option('doc').disabled()
+  subdir_done()
+endif
+
 if meson.is_cross_build()
     if get_option('doc').enabled()
         error('Documentation enabled but building the doc while cross building is not supported yet.')

subprojects/gst-plugins-bad/docs/meson.build

@@ -1,5 +1,9 @@
 build_hotdoc = false
 
+if get_option('doc').disabled()
+  subdir_done()
+endif
+
 if meson.is_cross_build()
     if get_option('doc').enabled()
         error('Documentation enabled but building the doc while cross building is not supported yet.')

subprojects/gst-plugins-base/docs/meson.build

@@ -1,5 +1,9 @@
 build_hotdoc = false
 
+if get_option('doc').disabled()
+  subdir_done()
+endif
+
 if meson.is_cross_build()
     if get_option('doc').enabled()
         error('Documentation enabled but building the doc while cross building is not supported yet.')

subprojects/gst-plugins-good/docs/meson.build

@@ -1,5 +1,9 @@
 build_hotdoc = false
 
+if get_option('doc').disabled()
+  subdir_done()
+endif
+
 if static_build
     if get_option('doc').enabled()
         error('Documentation enabled but not supported when building statically.')

subprojects/gst-plugins-ugly/docs/meson.build

@@ -1,5 +1,9 @@
 build_hotdoc = false
 
+if get_option('doc').disabled()
+  subdir_done()
+endif
+
 if meson.is_cross_build()
     if get_option('doc').enabled()
         error('Documentation enabled but building the doc while cross building is not supported yet.')

subprojects/gst-rtsp-server/docs/meson.build

@@ -1,5 +1,9 @@
 build_hotdoc = false
 
+if get_option('doc').disabled()
+  subdir_done()
+endif
+
 if meson.is_cross_build()
     if get_option('doc').enabled()
         error('Documentation enabled but building the doc while cross building is not supported yet.')

subprojects/gstreamer-vaapi/docs/meson.build

@@ -1,5 +1,9 @@
 build_hotdoc = false
 
+if get_option('doc').disabled()
+  subdir_done()
+endif
+
 if meson.is_cross_build()
     if get_option('doc').enabled()
         error('Documentation enabled but building the doc while cross building is not supported yet.')