Allow #pragma GCC in headers in conformtest

No "#pragma GCC" pragma allows macro-expansion of its arguments, so no
namespace issues arise from use of such pragmas in installed headers.
Ignore them in conformtest tests of header namespace.

Tested for x86_64, in conjunction with Paul's patch
<https://sourceware.org/pipermail/libc-alpha/2021-August/130571.html>
adding use of such pragmas to installed headers shared with gnulib.

conform/conformtest.py

@@ -624,6 +624,14 @@ class HeaderTests(object):
                     continue
                 if re.match(r'# [1-9]', line):
                     continue
+                if line.startswith('#pragma GCC '):
+                    # No GCC pragma uses macro expansion, so no
+                    # namespace issues arise from such pragmas.  (Some
+                    # pragmas not in the GCC namespace do macro-expand
+                    # their arguments and so could be affected by
+                    # macros defined by user code including the
+                    # header.)
+                    continue
                 match = re.match(r'#define (.*)', line)
                 if match:
                     self.check_token(bad_tokens, match.group(1))