Adding setup-hook for opensp, to set the sgml catalog files

This sets the SGML_CATALOG_FILES, similar to how libxml2 does the same with
XML_CATALOG_FILES.

I based the hook on the libxml2 one, and I followed the instructions about this
variable here:
http://docbook.sourceforge.net/release/dsssl/1.79/doc/install.html

pkgs/tools/text/sgml/opensp/compat.nix

@@ -3,13 +3,17 @@
 stdenv.mkDerivation {
   name = "sp-compat-${builtins.substring 7 100 opensp.name}";
 
-  buildCommand = ''
+  phases = [ "fixupPhase" "installPhase" ];
+
+  installPhase = ''
     mkdir -pv $out/bin
     for i in ${opensp}/bin/o*; do
       ln -sv $i $out/bin/''${i#${opensp}/bin/o}
     done
     '';
 
+  setupHook = opensp.setupHook;
+
   meta.description =
     "Compatibility wrapper for old programs looking for original sp programs";
 }

pkgs/tools/text/sgml/opensp/default.nix

@@ -13,6 +13,8 @@ stdenv.mkDerivation {
       docsrc/*.xml
   '';
 
+  setupHook = ./setup-hook.sh;
+
   buildInputs = [ xmlto docbook_xml_dtd_412 libxslt docbook_xsl ];
 
   meta = {

pkgs/tools/text/sgml/opensp/setup-hook.sh

@@ -0,0 +1,22 @@
+addSGMLCatalogs () {
+      if test -d $1/sgml/dtd; then
+          for i in $(find $1/sgml/dtd -name docbook.cat); do
+              export SGML_CATALOG_FILES="${SGML_CATALOG_FILES:+:}$i"
+          done
+      fi
+}
+
+if test -z "$sgmlHookDone"; then
+    sgmlHookDone=1
+
+    # Set http_proxy and ftp_proxy to a invalid host to prevent
+    # xmllint and xsltproc from trying to download DTDs from the
+    # network even when --nonet is not given.  That would be impure.
+    # (Note that .invalid is a reserved domain guaranteed not to
+    # work.)
+    export http_proxy=http://nodtd.invalid/
+    export ftp_proxy=http://nodtd.invalid/
+
+    export SGML_CATALOG_FILES
+    envHooks=(${envHooks[@]} addSGMLCatalogs)
+fi