gap: install libgap

There are some starts of a `make install`, but nothing complete yet.
Upstream now ships a `libgap` as a replacement of the custom one used by
sagemath.

pkgs/applications/science/math/gap/default.nix

@@ -107,7 +107,16 @@ stdenv.mkDerivation rec {
     popd
   '';
 
-  installPhase = ''
+  installTargets = [
+    "install-libgap"
+    "install-headers"
+  ];
+
+  # full `make install` is not yet implemented, just for libgap and headers
+  postInstall = ''
+    # Install config.h, which is not currently handled by `make install-headers`
+    cp gen/config.h "$out/include/gap"
+
     mkdir -p "$out/bin" "$out/share/gap/"
 
     mkdir -p "$out/share/gap"

pkgs/top-level/all-packages.nix

@@ -10806,17 +10806,31 @@ in
 
   libgadu = callPackage ../development/libraries/libgadu { };
 
+  # Deprecated since gap itself now ships with a library component. This is
+  # still necessary for sage 8.5 but will be removed once we switch to sage
+  # 8.6.
   gap-libgap-compatible = let
     version = "4r8p6";
     pkgVer = "2016_11_12-14_25";
   in
-    (gap.override { keepAllPackages = false; }).overrideAttrs (oldAttrs: {
+    (gap.override { packageSet = "minimal"; }).overrideAttrs (oldAttrs: {
       name = "libgap-${oldAttrs.pname}-${version}";
       inherit version;
       src = fetchurl {
         url = "https://www.gap-system.org/pub/gap/gap48/tar.bz2/gap${version}_${pkgVer}.tar.bz2";
         sha256 = "19n2p1mdg33s2x9rs51iak7rgndc1cwr56jyqnah0g1ydgg1yh6b";
       };
+      # libgap targets not yet available for 4r8p6
+      installPhase = ''
+        mkdir -p "$out/bin" "$out/share/gap/"
+
+        mkdir -p "$out/share/gap"
+        echo "Copying files to target directory"
+        cp -ar . "$out/share/gap/build-dir"
+
+        makeWrapper "$out/share/gap/build-dir/bin/gap.sh" "$out/bin/gap" \
+          --set GAP_DIR $out/share/gap/build-dir
+      '';
       patches = [
         # don't install any packages by default (needed for interop with libgap, probably obsolete  with 4r10
         (fetchpatch {