Try gcc-4.9 to fix compilation with clang-3.8.

toolset/setup/linux/frameworks/silicon.sh

@@ -1,6 +1,6 @@
 #!/bin/bash
 
-fw_depends clang-3.8 gcc-6
+fw_depends clang-3.8 gcc-4.9
 
 RETCODE=$(fw_exists ${IROOT}/silicon.installed)
 [ ! "$RETCODE" == 0 ] || { \
@@ -14,7 +14,7 @@ git clone https://github.com/matt-42/silicon.git
 cd silicon;
 # May 18th, 2016
 git checkout 73dac7f3c8dcd4f9c53713456e8b73165006e968
-CXX=clang++-3.8 ./install.sh $IROOT
+CC=clang CXX=clang++-3.8 ./install.sh $IROOT
 
 echo "" > $IROOT/silicon.installed