cmd/cgo: ignore unrecognized GCC warning group pragmas

CL 159859 causes build failure with old clang versions (3.4.1) on FreeBSD 10.3/10.4.

Update #29962
Update #27619

Change-Id: I78264ac5d8d17eeae89a982e89aac988eb22b286
Reviewed-on: https://go-review.googlesource.com/c/160777
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/cgo/out.go

@@ -777,6 +777,9 @@ func (p *Package) writeExports(fgo2, fm, fgcc, fgcch io.Writer) {
 	fmt.Fprintf(fgcc, "#include \"_cgo_export.h\"\n\n")
 
 	// We use packed structs, but they are always aligned.
+	// The pragmas and address-of-packed-member are not recognized as warning groups in clang 3.4.1, so ignore unknown pragmas first.
+	// remove as part of #27619 (all: drop support for FreeBSD 10).
+	fmt.Fprintf(fgcc, "#pragma GCC diagnostic ignored \"-Wunknown-pragmas\"\n")
 	fmt.Fprintf(fgcc, "#pragma GCC diagnostic ignored \"-Wpragmas\"\n")
 	fmt.Fprintf(fgcc, "#pragma GCC diagnostic ignored \"-Waddress-of-packed-member\"\n")
 
@@ -1478,6 +1481,10 @@ __cgo_size_assert(double, 8)
 extern char* _cgo_topofstack(void);
 
 /* We use packed structs, but they are always aligned.  */
+/* The pragmas and address-of-packed-member are not recognized as warning groups in clang 3.4.1, so ignore unknown pragmas first. */
+/* remove as part of #27619 (all: drop support for FreeBSD 10). */
+
+#pragma GCC diagnostic ignored "-Wunknown-pragmas"
 #pragma GCC diagnostic ignored "-Wpragmas"
 #pragma GCC diagnostic ignored "-Waddress-of-packed-member"