Update build & CI files to properly use new CI machine

The title says it all. I have to say your unconventional directory layout (in particular, not using a theories folder) makes it more annoying than it would have to be to update your build system. ;)

If you tell me there's currently no branch somewhere that would break, I'd be happy to introduce a theories folder for you. That would mostly involve git mv coq/ra coq/theories as well as moving Makefile and _CoqProject one folder up, i.e., directly into coq/.

coq/build/opam-ci.sh

@@ -4,24 +4,34 @@ set -e
 
 # Prepare OPAM configuration
 export OPAMROOT="$(pwd)/opamroot"
-export OPAMJOBS=16
+export OPAMJOBS="$((2*$CPU_CORES))"
 export OPAM_EDITOR="$(which false)"
 
 # Make sure we got a good OPAM
 test -d "$OPAMROOT" || (mkdir "$OPAMROOT" && opam init --no-setup -y)
 eval `opam conf env`
+if test $(find "$OPAMROOT/repo/package-index" -mtime +0); then
+    # last update was more than a day ago
+    opam update
+else
+    echo "[opam-ci] Not updating opam."
+fi
 test -d "$OPAMROOT/repo/coq-extra-dev" || opam repo add coq-extra-dev https://coq.inria.fr/opam/extra-dev -p 5
 test -d "$OPAMROOT/repo/coq-core-dev" || opam repo add coq-core-dev https://coq.inria.fr/opam/core-dev -p 5
 test -d "$OPAMROOT/repo/coq-released" || opam repo add coq-released https://coq.inria.fr/opam/released -p 10
-opam update
-opam install ocamlfind -y # Remove this once the Coq crew fixed their package...
 
 # Install fixed versions of some dependencies
 echo
-for PIN in "${@}"
-do
-    echo "Applying pin: $PIN"
-    opam pin add $PIN -k version -y
+while (( "$#" )); do # while there are arguments left
+    PACKAGE="$1" ; shift
+    VERSION="$1" ; shift
+    # Check if the pin is already set
+    if opam pin list | fgrep "$PACKAGE.$VERSION " > /dev/null; then
+        echo "[opam-ci] $PACKAGE already pinned to $VERSION"
+    else
+        echo "[opam-ci] Pinning $PACKAGE to $VERSION"
+        opam pin add "$PACKAGE" "$VERSION" -k version -y
+    fi
 done
 
 # Install build-dependencies