meson: add source dir to include_directories automatically

Quite often we need to specify include_directories('.'). Normally, meson
does this automatically, but we specify implicit_include_directories : false,
see 30d20907bd for an explanation.
Passing the include_directories argument through the template layers was rather
cumbersome. Let's simplify things by always including the directory of the
first source file. This makes the definitions simpler, at the cost of having an
unnecessary include directory in the list in some cases.

(Tests are excluded from this change, because they happen to not need this, but
also because some tests have source[0] which is a custom target, and we'd need
newer meson to support that.)

meson.build

@@ -2334,7 +2334,6 @@ else
         }
 endif
 fuzz_additional_kwargs += {
-        'include_directories' : include_directories('src/fuzz'),
         'c_args' : test_cflags,
 }
 
@@ -2565,9 +2564,9 @@ foreach dict : executables
 
         kwargs = {}
         foreach key, val : dict
-                if key in ['name', 'dbus', 'public', 'conditions',
-                           'type', 'suite', 'timeout', 'parallel',
-                           'objects', 'sources', 'extract']
+                if key in ['name', 'dbus', 'public', 'conditions', 'type', 'suite',
+                           'timeout', 'parallel', 'objects', 'sources', 'extract',
+                           'include_directories']
                         continue
                 endif
 
@@ -2578,15 +2577,15 @@ foreach dict : executables
                 kwargs += { key : [ kwargs.get(key, []), val ]}
         endforeach
 
+        include_directories = dict['include_directories']
+        if not is_test
+                include_directories += fs.parent(exe_sources[0])
+        endif
+
         foreach val : dict.get('objects', [])
                 obj = objects_by_name[val]
-                kwargs += {
-                        'objects' : obj['objects'],
-                        'include_directories' : [
-                                kwargs.get('include_directories', []),
-                                obj['include_directories'],
-                        ],
-                }
+                kwargs += { 'objects' : obj['objects'] }
+                include_directories += obj['include_directories']
         endforeach
 
         if is_test
@@ -2597,6 +2596,7 @@ foreach dict : executables
         endif
 
         if is_fuzz
+                include_directories += include_directories('src/fuzz')
                 foreach key, val : fuzz_additional_kwargs
                         if key == 'sources'
                                 exe_sources += val
@@ -2611,6 +2611,7 @@ foreach dict : executables
                 sources : exe_sources,
                 kwargs : kwargs,
                 implicit_include_directories : false,
+                include_directories : include_directories,
         )
 
         executables_by_name += { name : exe }