Linux: Pin NodeJS to 8.x, fix watchman install

Summary:
- On newer versions of Ubuntu, NodeJS 10.x is in the official packages, so apt will upgrade the PPA-installed 8.x version to 10.x the next time upgrades are run. This change adds an apt preferences file that pins the `nodejs` package at version 8.x.
- The watchman installation was previously broken on new installs, because the `ka-clone` executable doesn't get installed until after the Linux-specific installation is done. This commit changes the previous call to `kaclone_repo` to bare git commands, so it will work at all times.
- The watchman build previously didn't work on some Ubuntu machines, because there were two missing dependencies. This commit adds them to the `apt-get install` command run before building.
- Newer versions of GCC turn certain warnings into fatal errors, and watchman hasn't yet been updated to avoid those errors. As a workaround, we pass `--enable-lenient` to the configure command, so those warnings won't break the compile.

Test Plan:
- The scripts were executed on fresh Ubuntu 18.04 and 19.04 installations, as well as my previously-configured Ubuntu 19.04 machine.
- MacOS was not tested, because the changes are local to `linux-setup.sh`.

Reviewers: benkraft, colin, kevinb

Reviewed By: benkraft

Subscribers: csilvers

Differential Revision: https://phabricator.khanacademy.org/D54498

linux-setup.sh

@@ -65,6 +65,14 @@ deb https://deb.nodesource.com/node_8.x `lsb_release -c -s` main
 deb-src https://deb.nodesource.com/node_8.x `lsb_release -c -s` main
 EOF
         sudo chmod a+rX /etc/apt/sources.list.d/nodesource.list
+
+        # Pin nodejs to 8.x, otherwise apt will update it to 10.x on newer
+        # Ubuntu versions
+        cat <<EOF | sudo tee /etc/apt/preferences.d/nodejs
+Package: nodejs
+Pin: version 8.*
+Pin-Priority: 999
+EOF
         updated_apt_repo=yes
     fi
 
@@ -218,14 +226,21 @@ install_protoc() {
 install_watchman() {
     if ! which watchman ; then
         update "Installing watchman..."
-        kaclone_repo https://github.com/facebook/watchman.git "$REPOS_DIR/"
+        builddir="$DEVTOOLS_DIR/watchman/"
+        if [ ! -d "$builddir" ]; then
+            mkdir -p "$builddir"
+            git clone https://github.com/facebook/watchman.git "$builddir"
+        fi
+
         (
             # Adapted from https://medium.com/@saurabh.friday/install-watchman-on-ubuntu-18-04-ba23c56eb23a
-            cd "$REPOS_DIR/watchman"
-            sudo apt-get install -y autoconf automake build-essential python-dev
+            cd "$builddir"
+            sudo apt-get install -y autoconf automake build-essential python-dev libtool libssl-dev
             git checkout tags/v4.9.0
             ./autogen.sh
-            ./configure
+            # --enable-lenient is required for newer versions of GCC, which is
+            # stricter with certain constructs.
+            ./configure --enable-lenient
             make
             sudo make install
         )