configure.ac: check if lzma has MT support, fix deb build/etc.

configure.ac

@@ -175,6 +175,8 @@ AC_SUBST(HAVE_SODIUM, [$have_sodium])
 
 # Look for liblzma, a required dependency.
 PKG_CHECK_MODULES([LIBLZMA], [liblzma], [CXXFLAGS="$LIBLZMA_CFLAGS $CXXFLAGS"])
+AC_CHECK_LIB([lzma], [lzma_stream_encoder_mt],
+  [AC_DEFINE([HAVE_LZMA_MT], [1], [xz multithreaded compression support])])
 
 
 # Look for libbrotli{enc,dec}, optional dependencies

src/libutil/compression.cc

@@ -192,6 +192,7 @@ struct XzSink : CompressionSink
     XzSink(Sink & nextSink, const bool parallel) : nextSink(nextSink)
     {
         lzma_ret ret;
+#ifdef HAVE_LZMA_MT
         if (parallel) {
             lzma_mt mt_options = {};
             mt_options.flags = 0;
@@ -208,6 +209,7 @@ struct XzSink : CompressionSink
             ret = lzma_stream_encoder_mt(
                 &strm, &mt_options);
         } else
+#endif
             ret = lzma_easy_encoder(
                 &strm, 6, LZMA_CHECK_CRC64);