[feature] configure: generating a list of available optional tags, fo…

…r plugins building

config.mli

@@ -45,6 +45,10 @@ val libnatpmp : (string*string*string) option (** name of the lib, lib dir, incl
 
 val miniupnpc : (string*string*string) option (** name of the lib, lib dir, include dir *)
 
+(** list all of the optional tags that have been detected available (eg
+    "camlidl", "libnatpmp", "miniupnpc", ...) *)
+val available : string list
+
 (** are we compiling in release or debug mode ? In release mode,
     assertions and debug variables are disabled *)
 val is_release : bool

configure

@@ -546,6 +546,12 @@ camlopt() {
     fi
 }
 
+TAGS_LIST=""
+for tag in camlidl libnatpmp miniupnpc; do
+    hasvar=HAS_${tag^^}
+    if [ -n "${!hasvar}" ]; then TAGS_LIST="$TAGS_LIST\"$tag\"; "; fi
+done
+
 # See config.mli for the documentation and spec of these variables
 cat >config.ml <<EOF
 let prefix = "$PREFIX"
@@ -560,6 +566,8 @@ let camlidl = $(camlopt "$HAS_CAMLIDL" '"'"$CAMLIDL"'"')
 let libnatpmp = $(camlopt "$HAS_LIBNATPMP" "$(camlstrtuple "${LIBNATPMP[@]}")")
 let miniupnpc = $(camlopt "$HAS_MINIUPNPC" "$(camlstrtuple "${MINIUPNPC[@]}")")
 
+let available = [ $TAGS_LIST]
+
 let is_release = $(camlbool "$IS_RELEASE")
 type os = Linux | Mac | Win32 | Cygwin
 let os =