Add a meson option for a local public suffix list

gen-public-suffix.py downloads public_suffix_list.dat at build time and
only falls back to a few hardcoded Linux paths when that fails. Package
builds usually run without network access, and on FreeBSD the list is
installed under /usr/local/share/public_suffix_list, so the build dies
with "unable to load public suffix list".

Add a public-suffix-list option that takes the path of a local copy.
When it is set the generator reads just that file and never goes
online, which also keeps the generated table reproducible.

src/common/gen-public-suffix.py

@@ -38,16 +38,16 @@ def main():
     if len(sys.argv) not in (2, 3):
         raise SystemExit("usage: gen-public-suffix.py <output> [source]")
     output = Path(sys.argv[1])
-    sources = []
     if len(sys.argv) == 3:
-        sources.append(Path(sys.argv[2]))
-    sources.extend(
-        [
-            Path(__file__).with_name("public_suffix_list.dat"),
-            Path("/usr/share/publicsuffix/public_suffix_list.dat"),
-            Path("/app/share/publicsuffix/public_suffix_list.dat"),
-        ]
-    )
+        # An explicit source is authoritative, don't go online behind its back.
+        rules = parse_rules(Path(sys.argv[2]).read_text(encoding="utf-8"))
+        emit_header(str(output), rules)
+        return
+    sources = [
+        Path(__file__).with_name("public_suffix_list.dat"),
+        Path("/usr/share/publicsuffix/public_suffix_list.dat"),
+        Path("/app/share/publicsuffix/public_suffix_list.dat"),
+    ]
     data = None
     for url in URLS:
         try:

src/common/meson.build

@@ -110,9 +110,15 @@ make_te = find_program('make-te.py')
 
 python3 = find_program('python3', required: true)
 
+public_suffix_cmd = [python3, files('gen-public-suffix.py'), '@OUTPUT@']
+public_suffix_list = get_option('public-suffix-list')
+if public_suffix_list != ''
+  public_suffix_cmd += public_suffix_list
+endif
+
 public_suffix_data = custom_target('public_suffix_data_h',
   output: 'public_suffix_data.h',
-  command: [python3, files('gen-public-suffix.py'), '@OUTPUT@']
+  command: public_suffix_cmd
 )
 
 emoji_data_table = custom_target('emoji_data_table_h',