Set SUBDIRS equal to autoconf variable RTLIBSUBSYS

Otherwise, the make "dist" target fails if Gambit Universal IDE
(guide) is disabled by configure.

Assuming configure does not enable guide, how to repeat:

````
$ git clone git://github.com/feeley/gambit.git
$ cd gambit
$ ./configure --enable-single-host --enable-c-opt --enable-gcc-opts --enable-multiple-versions
$ make bootstrap
$ make bootclean
$ make
$ make dist
   :
   :
making dist in guide
mkdir ../../gambc-v4_6_6/lib/guide
chmod 777 ../../gambc-v4_6_6/lib/guide
  Copying distribution files:
    lib/guide/makefile.in
    lib/guide/guidepro.in
    lib/guide/_guide.scm
    lib/guide/_guide#.scm
    lib/guide/guideuicodeformat.cpp
    lib/guide/guideuirepl.cpp
    lib/guide/guideuihighlighter.cpp
    lib/guide/guideuihighlighterscheme.cpp
    lib/guide/guideuischeme.cpp
    lib/guide/guideuiconsoleinfo.cpp
    lib/guide/guideuitextedit.cpp
    lib/guide/guideuicont.cpp
    lib/guide/guideuienv.cpp
    lib/guide/guideuiinspector.cpp
    lib/guide/guide.cpp
    lib/guide/guideuicodeformat.h
    lib/guide/guideuirepl.h
    lib/guide/guideuitableitem.h
    lib/guide/guideuihighlighter.h
    lib/guide/guideuihighlighterscheme.h
    lib/guide/guideuischeme.h
    lib/guide/guideuiconsoleinfo.h
    lib/guide/guideuitextedit.h
    lib/guide/guideuicont.h
    lib/guide/guideuienv.h
    lib/guide/guideuiinspector.h
    lib/guide/_guide.h
    lib/guide/guide.h
    lib/guide/guideuimainwindow.ui
    lib/guide/guideuifileeditor.ui
    lib/guide/guideuiformatchooser.ui
    lib/guide/guideuisearchdialog.ui
    lib/guide/guideuiabout.ui
    lib/guide/_guide.c
cp: _guide.c: No such file or directory
make[2]: *** [dist-pre] Error 1
make[1]: *** [dist-recursive] Error 1
make: *** [dist-recursive] Error 1
````