install: add support for env var and system type set on configure

Add support to define environment variable and build system type as
`configure` script arguments to better match GNU configuration
specification.

configure

@@ -632,8 +632,16 @@ for arg in "$@"; do
       # GNU Configuration specification
       # (https://www.gnu.org/prep/standards/html_node/Configuration.html)
       echo_warning "Option \`$arg' ignored" ;;
-   *)
+   -*)
       echo_error "Unrecognized option \`$arg'" 1;;
+   *=*)
+      echo -e "Export \`$arg' in environment" ;
+      export $arg ;;
+   *)
+      # type of system to build the program for (not needed here but useful
+      # for true autoconf configure scripts below
+      compatarglist+="$arg " ;
+      libarglist+="$arg " ;;
    esac
 done