properly use the undocumented libtool shrext_cmds variable

shrext_cmds is actually a quoted set of commands to generate the
shared library extension, not the actual shrext itself (regardless of
it initially being set directly to .so as a default).

Author: djerius

build-aux/boost.m4

@@ -430,6 +430,7 @@ dnl start the for loops).
     [AC_MSG_ERROR([cannot compile a test that uses Boost $1])])
   ac_objext=$boost_save_ac_objext
   boost_failed_libs=
+  eval eval _boost_shrext=$shrext_cmds
 # Don't bother to ident the following nested for loops, only the 2
 # innermost ones matter.
 for boost_lib_ in $2; do
@@ -474,7 +475,7 @@ for boost_rtopt_ in $boost_rtopt '' -d; do
         (*?*:*s*) # Yes (Non empty boost_ldpath + s in rt opt)
           Boost_lib_LIBS="$boost_ldpath/lib$boost_lib.$libext" ;;
         (*) # No:
-          Boost_lib_LIBS="$boost_ldpath/lib$boost_lib$shrext_cmds" ;;
+          Boost_lib_LIBS="$boost_ldpath/lib$boost_lib$_boost_shrext" ;;
       esac
       # Don't waste time with libraries that don't exist
       test -e "$Boost_lib_LIBS" || continue