Few more (final?) fixes for Docker build

Dockerfile

@@ -1,4 +1,6 @@
 FROM emscripten/emsdk:2.0.26
+# Update to tot to include EM_ASYNC_JS; remove once 2.0.27 is out:
+RUN emsdk install tot
 RUN apt-get update && apt-get install -qqy autoconf autopoint pkg-config
 WORKDIR /src
 ENV EM_CACHE /src/deps/.emcache

Makefile

@@ -1,7 +1,7 @@
 SYSROOT = $(EM_CACHE)/sysroot
 
-# Add custom sysroot to autoconf macro search
-# (we need this because we use custom local copy of libtool)
+# Add custom sysroot to library & macro search paths.
+export LDFLAGS += -L$(SYSROOT)/lib
 export ACLOCAL_PATH := $(SYSROOT)/share/aclocal:$(ACLOCAL_PATH)
 
 # Common linking flags for all targets.