Add pkg-config system dependency (PR #499)

json-data-encoding indirectly depends on it and the Docker builds have been failing ever since.

Dockerfile

@@ -8,7 +8,7 @@ COPY --chown=opam Makefile make.sh goblint.opam goblint.opam.locked /home/opam/a
 WORKDIR /home/opam/analyzer/
 # TODO: use opam depext
 RUN sudo apt-get update \
-    && sudo apt-get install -y libgmp-dev libmpfr-dev
+    && sudo apt-get install -y libgmp-dev libmpfr-dev pkg-config
 # update local opam repository because base image may be outdated
 RUN cd /home/opam/opam-repository \
     && git pull origin master \

README.md

@@ -12,7 +12,7 @@ Both for using an up-to-date version of Goblint or developing it, the best way i
 
 ### Linux
 1. Install [opam](https://opam.ocaml.org/doc/Install.html).
-2. Make sure the following are installed: `git patch m4 autoconf libgmp-dev libmpfr-dev`.
+2. Make sure the following are installed: `git patch m4 autoconf libgmp-dev libmpfr-dev pkg-config`.
 3. Run `make setup` to install OCaml and dependencies via opam.
 4. Run `make` to build Goblint itself.
 5. Run `make install` to install Goblint into the opam switch for usage via switch's `PATH`.

make.sh

@@ -72,7 +72,7 @@ rule() {
         opam upgrade -y $(opam list --pinned -s)
       }
     ;; setup)
-      echo "Make sure you have the following installed: opam >= 2.0.0, git, patch, m4, autoconf, libgmp-dev, libmpfr-dev"
+      echo "Make sure you have the following installed: opam >= 2.0.0, git, patch, m4, autoconf, libgmp-dev, libmpfr-dev, pkg-config"
       echo "For the --html output you also need: javac, ant, dot (graphviz)"
       echo "For running the regression tests you also need: ruby, gem, curl"
       echo "For reference see ./Dockerfile or ./scripts/travis-ci.sh"