build: Also warn about header-only build targets

Build targets that do not contain any actual source files and only
headers should just be as affected as build targets with no sources
specified whatsoever, so the 'Build target has no sources.' warning
should be thrown for them too.

Author: sp1ritCS

mesonbuild/build.py

@@ -758,8 +758,8 @@ def __init__(
         self.link(link_targets)
         self.link_whole(link_whole_targets)
 
-        if not any([self.sources, self.generated, self.objects, self.link_whole_targets, self.structured_sources,
-                    kwargs.pop('_allow_no_sources', False)]):
+        if not any([[src for src in self.sources if not is_header(src)], self.generated, self.objects,
+                    self.link_whole_targets, self.structured_sources, kwargs.pop('_allow_no_sources', False)]):
             mlog.warning(f'Build target {name} has no sources. '
                          'This was never supposed to be allowed but did because of a bug, '
                          'support will be removed in a future release of Meson')