Add support for thin ar archives (#14324)

Fixes: #14261

Author: sbc100

tests/test_other.py

@@ -7622,6 +7622,13 @@ def test_archive_non_objects(self):
     self.run_process([EMAR, 'crS', '--format=gnu', 'libfoo.a', 'file.txt', 'hello_world.o'])
     self.run_process([EMCC, test_file('hello_world.c'), 'libfoo.a'])
 
+  def test_archive_thin(self):
+    self.run_process([EMCC, '-c', test_file('hello_world.c')])
+    # The `T` flag means "thin"
+    self.run_process([EMAR, 'crT', 'libhello.a', 'hello_world.o'])
+    self.run_process([EMCC, 'libhello.a'])
+    self.assertContained('hello, world!', self.run_js('a.out.js'))
+
   def test_flag_aliases(self):
     def assert_aliases_match(flag1, flag2, flagarg, extra_args=[]):
       results = {}

tools/building.py

@@ -1287,7 +1287,7 @@ def is_ar(filename):
     if _is_ar_cache.get(filename):
       return _is_ar_cache[filename]
     header = open(filename, 'rb').read(8)
-    sigcheck = header == b'!<arch>\n'
+    sigcheck = header in (b'!<arch>\n', b'!<thin>\n')
     _is_ar_cache[filename] = sigcheck
     return sigcheck
   except Exception as e: