[libc++] Improves pragma system_header test.

[Mordante]

Summary

The number of spaces between # and pragma can differ due to
different indention levels in the preprocessor directives. Therefore
allow any number of spaces.

The test used to put an exclamation mark in its diagnostic. This adds
little benefit and only makes it harder to copy the offending filename.
As drive-by this exclamation mark has been removed.

libcxx/test/libcxx/lint/lint_headers.sh.py

return (
         os.path.basename(path) == '__config' or
         os.path.basename(path) == '__config_site.in' or
         not os.path.isfile(path)
     )
 
 
 def check_for_pragma_GCC_system_header(pretty_fname, lines):
     if pretty_fname not in ['__undef_macros']:
-        if '#  pragma GCC system_header\n' not in lines:
-            print('FAILED TO FIND #  pragma GCC system_header in libcxx/include/%s!' % pretty_fname)
-            return False
+        for line in lines:
+            if re.match('# *pragma GCC system_header\n', line):
+                return True
+        print('FAILED TO FIND #  pragma GCC system_header in libcxx/include/%s' % pretty_fname)
     return True
 
 
 if __name__ == '__main__':
     libcxx_test_libcxx_lint = os.path.dirname(os.path.abspath(__file__))
     libcxx_include = os.path.abspath(os.path.join(libcxx_test_libcxx_lint, '../../../include'))
     assert os.path.isdir(libcxx_include)