use libbrotli directly when available

* Look for both 'brotli' and 'bro' as external command,
  since upstream has renamed it in newer versions.
  If neither are found, current runtime behavior
  is preserved: try to find 'bro' on PATH.
* Limit amount handed to BrotliEncoderCompressStream
  to ensure interrupts are processed in a timely manner.
  Testing shows negligible performance impact.
  (Other compression sinks don't seem to require this)

configure.ac

@@ -127,7 +127,7 @@ NEED_PROG(gzip, gzip)
 NEED_PROG(xz, xz)
 AC_PATH_PROG(dot, dot)
 AC_PATH_PROG(pv, pv, pv)
-AC_PATH_PROG(bro, bro, bro)
+AC_PATH_PROGS(brotli, brotli bro, bro)
 AC_PATH_PROG(lsof, lsof, lsof)
 
 
@@ -176,6 +176,13 @@ AC_SUBST(HAVE_SODIUM, [$have_sodium])
 PKG_CHECK_MODULES([LIBLZMA], [liblzma], [CXXFLAGS="$LIBLZMA_CFLAGS $CXXFLAGS"])
 
 
+# Look for libbrotli{enc,dec}, optional dependencies
+PKG_CHECK_MODULES([LIBBROTLI], [libbrotlienc libbrotlidec],
+  [AC_DEFINE([HAVE_BROTLI], [1], [Whether to use libbrotli.])
+   CXXFLAGS="$LIBBROTLI_CFLAGS $CXXFLAGS"]
+   have_brotli=1], [have_brotli=])
+AC_SUBST(HAVE_BROTLI, [$have_brotli])
+
 # Look for libseccomp, required for Linux sandboxing.
 if test "$sys_name" = linux; then
   PKG_CHECK_MODULES([LIBSECCOMP], [libseccomp],