Make ext_skel create a Makefile.in set up to handle shared extension

support automatically
@ Make ext_skel create a Makefile.in set up to handle shared extension
@ support automatically (Rasmus)

ext/ext_skel

@@ -144,6 +144,7 @@ cat >Makefile.in <<eof
 LTLIBRARY_NAME        = lib$extname.la
 LTLIBRARY_SOURCES     = $extname.c
 LTLIBRARY_SHARED_NAME = $extname.la
+LTLIBRARY_SHARED_LIBADD  = \$(${EXTNAME}_SHARED_LIBADD)
 
 include \$(top_srcdir)/build/dynlib.mk
 eof