Bug 1677715 - Add a way of setting TK_CFLAGS in a gn generated moz.build file r=firefox-build-system-reviewers,mhentges,ng

This adds write_mozbuild_variables to the GnProjectData that can be used to
configure code generation of moz.build files. This is done to support adding
CONFIG["TK_CFLAGS"] to INCLUDES for gtk builds, which is necessary to support
pipewire.

There is likely a more elegant way of handling this, but since we plan to
remove code generation of moz.build files in the near future anyway, I'm not
sure it is worth investing more time than a minimal amount of time in this.

Differential Revision: https://phabricator.services.mozilla.com/D97427

python/mozbuild/mozbuild/frontend/context.py

@@ -2405,6 +2405,7 @@ VARIABLES = {
                 "non_unified_sources": StrictOrderingOnAppendList,
                 "mozilla_flags": list,
                 "gn_target": six.text_type,
+                "write_mozbuild_vars": dict,
             }
         ),
         list,
@@ -2419,6 +2420,8 @@ VARIABLES = {
             - mozilla_flags, a set of flags that if present in the gn config
               will be mirrored to the resulting mozbuild configuration.
             - gn_target, the name of the target to build.
+            - write_mozbuild_vars, a dictionary containing variables to control
+              code generation of moz.build files.
         """,
     ),
     "SPHINX_TREES": (

python/mozbuild/mozbuild/frontend/data.py

@@ -1437,3 +1437,4 @@ class GnProjectData(ContextDerived):
         self.gn_sandbox_variables = gn_dir_attrs.sandbox_vars
         self.mozilla_flags = gn_dir_attrs.mozilla_flags
         self.gn_target = gn_dir_attrs.gn_target
+        self.write_mozbuild_variables = gn_dir_attrs.write_mozbuild_vars

python/mozbuild/mozbuild/gn_processor.py

@@ -434,7 +434,13 @@ def find_common_attrs(config_attributes):
 
 
 def write_mozbuild(
-    config, srcdir, output, non_unified_sources, gn_config_files, mozilla_flags
+    config,
+    srcdir,
+    output,
+    non_unified_sources,
+    gn_config_files,
+    mozilla_flags,
+    write_mozbuild_variables,
 ):
 
     all_mozbuild_results = []
@@ -473,6 +479,13 @@ def write_mozbuild(
             mb.write("\n")
             mb.write(generated_header)
 
+            try:
+                if relsrcdir in write_mozbuild_variables["INCLUDE_TK_CFLAGS_DIRS"]:
+                    mb.write('if CONFIG["MOZ_WIDGET_TOOLKIT"] == "gtk":\n')
+                    mb.write('    CXXFLAGS += CONFIG["TK_CFLAGS"]\n')
+            except KeyError:
+                pass
+
             all_args = [args for args, _ in configs]
 
             # Start with attributes that will be a part of the mozconfig
@@ -653,6 +666,7 @@ class GnMozbuildWriterBackend(BuildBackend):
                     obj.non_unified_sources,
                     gn_config_files,
                     obj.mozilla_flags,
+                    obj.write_mozbuild_variables,
                 )
             else:
                 print(