gettext-runtime: use @SHELL@ for the SHELL variable definition

The gettext-runtime/Makefile.in uses an hardcoded SHELL = /bin/sh
variable. This causes problems at build time, because libtool is then
called with the 'sh' shell, which does not support the += construct
that the libtool script is using.

Instead, this Makefile.in should be definining SHELL = @SHELL@, just
like all other Makefile.in of gettext.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Index: b/gettext-runtime/intl/Makefile.in
===================================================================
--- a/gettext-runtime/intl/Makefile.in
+++ b/gettext-runtime/intl/Makefile.in
@@ -17,7 +17,7 @@
 PACKAGE = @PACKAGE@
 VERSION = @VERSION@
 
-SHELL = /bin/sh
+SHELL = @SHELL@
 
 srcdir = @srcdir@
 top_srcdir = @top_srcdir@