gcc/amd64 head build error after r281316

Hi,
the r281316 commit introduces the following lines
which break compilation with gcc on amd64 (as far as i know
immintrin.h is only available in our clang).
If there are no objections I'd like to add a further check
for the use of clang, see attached patch

Index: /home/luigi/FreeBSD/head/lib/liblzma/config.h
===================================================================
--- /home/luigi/FreeBSD/head/lib/liblzma/config.h       (revision 285281)
+++ /home/luigi/FreeBSD/head/lib/liblzma/config.h       (working copy)
_at__at_ -150,7 +150,7 _at__at_
 #define HAVE_ICONV 1
 
 /* Define to 1 if you have the <immintrin.h> header file. */
-#if defined(__FreeBSD__) && defined(__amd64__)
+#if defined(__clang__) && defined(__FreeBSD__) && defined(__amd64__)
 #define HAVE_IMMINTRIN_H 1
 #endif


cheers
luigi