Ticket #4459: usage of 'sed' in build system/makefiles is not portable.

configure: add explicit sed detection

Actually SED variable is set automatically by LT_INIT currently.
Having explicit check should help avoiding problems if/when LT_INIT
changed.

Signed-off-by: Karlson2k (Evgeny Grin) <k2k@narod.ru>
Signed-off-by: Andrew Borodin <aborodin@vmail.ru>

configure.ac

@@ -71,6 +71,7 @@ dnl ############################################################################
 dnl Check for programs
 dnl ############################################################################
 
+AC_PROG_SED
 PKG_PROG_PKG_CONFIG
 AC_PROG_INSTALL
 AC_PROG_LN_S