tar: add IF_FEATURE_* checks

A following linking error was observed:
| ==========
| archival/lib.a(tar.o): In function `tar_main':
| archival/tar.c:1168: undefined reference to `unpack_Z_stream'
| archival/tar.c:1168: undefined reference to `unpack_Z_stream'
| ld: busybox_unstripped: hidden symbol `unpack_Z_stream' isn't defined
| ld: final link failed: Bad value

this happened with clang compiler, with the following configs:
| CONFIG_TAR=y
| # CONFIG_FEATURE_SEAMLESS_Z is not set

which can be fixed by adding IF_FEATURE_* checks in.

Signed-off-by: Ming Liu <peter.x.liu@external.atlascopco.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/archival/tar.c b/archival/tar.c
index b70e00a..7598b71 100644
--- a/archival/tar.c
+++ b/archival/tar.c
@@ -1216,21 +1216,26 @@
 		USE_FOR_MMU(IF_DESKTOP(long long) int FAST_FUNC (*xformer)(transformer_state_t *xstate);)
 		USE_FOR_NOMMU(const char *xformer_prog;)
 
-		if (opt & OPT_COMPRESS)
-			USE_FOR_MMU(xformer = unpack_Z_stream;)
+		if (opt & OPT_COMPRESS) {
+			USE_FOR_MMU(IF_FEATURE_SEAMLESS_Z(xformer = unpack_Z_stream;))
 			USE_FOR_NOMMU(xformer_prog = "uncompress";)
-		if (opt & OPT_GZIP)
-			USE_FOR_MMU(xformer = unpack_gz_stream;)
+		}
+		if (opt & OPT_GZIP) {
+			USE_FOR_MMU(IF_FEATURE_SEAMLESS_GZ(xformer = unpack_gz_stream;))
 			USE_FOR_NOMMU(xformer_prog = "gunzip";)
-		if (opt & OPT_BZIP2)
-			USE_FOR_MMU(xformer = unpack_bz2_stream;)
+		}
+		if (opt & OPT_BZIP2) {
+			USE_FOR_MMU(IF_FEATURE_SEAMLESS_BZ2(xformer = unpack_bz2_stream;))
 			USE_FOR_NOMMU(xformer_prog = "bunzip2";)
-		if (opt & OPT_LZMA)
-			USE_FOR_MMU(xformer = unpack_lzma_stream;)
+		}
+		if (opt & OPT_LZMA) {
+			USE_FOR_MMU(IF_FEATURE_SEAMLESS_LZMA(xformer = unpack_lzma_stream;))
 			USE_FOR_NOMMU(xformer_prog = "unlzma";)
-		if (opt & OPT_XZ)
-			USE_FOR_MMU(xformer = unpack_xz_stream;)
+		}
+		if (opt & OPT_XZ) {
+			USE_FOR_MMU(IF_FEATURE_SEAMLESS_XZ(xformer = unpack_xz_stream;))
 			USE_FOR_NOMMU(xformer_prog = "unxz";)
+		}
 
 		fork_transformer_with_sig(tar_handle->src_fd, xformer, xformer_prog);
 		/* Can't lseek over pipes */