install-deps.sh: use GCC-7 on xenial also

Signed-off-by: Kefu Chai <kchai@redhat.com>
(cherry picked from commit 8a0935988f)

install-deps.sh

@@ -31,10 +31,19 @@ function ensure_decent_gcc_on_deb {
     # new enough
     local old=$(gcc -dumpversion)
     local new=$1
-    if dpkg --compare-versions $old ge 5.1; then
+    if dpkg --compare-versions $old ge 7.0; then
 	return
     fi
 
+    if [ ! -f /usr/bin/gcc-${old} ]; then
+       case $(lsb_release --short --codename) in
+           trusty)
+               old=4.8;;
+           xenial)
+               old=5;;
+       esac
+    fi
+    $SUDO update-alternatives --remove-all gcc || true
     $SUDO update-alternatives \
 	 --install /usr/bin/gcc gcc /usr/bin/gcc-${new} 20 \
 	 --slave   /usr/bin/g++ g++ /usr/bin/g++-${new}
@@ -139,7 +148,7 @@ else
         $SUDO apt-get install -y lsb-release devscripts equivs
         $SUDO apt-get install -y dpkg-dev
         case "$VERSION" in
-            *Trusty*)
+            *Trusty*|*Xenial*)
                 $SUDO apt-get install -y software-properties-common
                 $SUDO add-apt-repository ppa:ubuntu-toolchain-r/test
                 $SUDO apt-get update