build: Optionally syntax-check sources

As a pure javascript project, building is really just a glorified
copy operation, so success doesn't even indicate that sources are
syntactically correct (a.k.a. "compile-tested"). We can at least
get some minimal testing by performing some basic syntax checking
when SpilderMonkey's JS shell is available.

Fixes https://gitlab.gnome.org/GNOME/gnome-shell-extensions/issues/32

extensions/meson.build

@@ -1,8 +1,9 @@
 extension_schemas = []
+js_sources = extensionlib
 
 metadata_name = 'metadata.json'
 
-foreach e : enabled_extensions
+foreach e : all_extensions
   uuid = e + '@gnome-shell-extensions.gcampax.github.com'
 
   metadata_conf = configuration_data()
@@ -18,11 +19,27 @@ foreach e : enabled_extensions
 
   subdir(e)
 
-  install_data (extension_sources + extension_data + extensionlib,
-    install_dir: join_paths(extensiondir, uuid)
-  )
+  js_sources += extension_sources
+
+  if (enabled_extensions.contains(e))
+    install_data (extension_sources + extension_data + extensionlib,
+      install_dir: join_paths(extensiondir, uuid)
+    )
+  endif
 endforeach
 
 install_data (extension_schemas,
   install_dir: schemadir
 )
+
+foreach js_source : js_sources
+  if (js52.found())
+    path_array = '@0@'.format(js_source).split('/')
+    name = join_paths(path_array[-2], path_array[-1])
+
+    test('Checking syntax of ' + name, js52,
+      args: ['-s', '-c', js_source],
+      workdir: meson.current_source_dir()
+    )
+  endif
+endforeach

meson.build

@@ -22,6 +22,7 @@ xsessiondir = join_paths(datadir, 'xsession')
 
 extensionlib = files('lib/convenience.js')
 
+js52 = find_program('js52', required: false)
 sassc = find_program('sassc', required: false)
 
 ver_arr = meson.project_version().split('.')