[smuxi/stable] configure.ac: check if shell supports +=

[Mirco M. M. Bauer <mmmbauer src gnome org>]

commit e094b140cdfc4b68a3549a4a98c7520d852094b2
Author: Mirco Bauer <meebey meebey net>
Date:   Fri Oct 9 21:26:12 2015 +0200

    configure.ac: check if shell supports +=
    
    On systems with non-bash /bin/sh appending variables using += will fail. The
    issue is that the configure script continues if it doesn't. This led to a
    failing build in some obscure place because the variable wasn't appended. So we
    bail now in the configure script early and give the user a hint how to fix it.

 configure.ac |   10 ++++++++++
 1 files changed, 10 insertions(+), 0 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 7f9b1cc..d70659e 100644
--- a/configure.ac
+++ b/configure.ac
@@ -16,6 +16,16 @@ AC_DISABLE_STATIC
 AC_PROG_LIBTOOL
 #LT_INIT([disable-static])
 
+AC_MSG_CHECKING([whether the shell understands "+="])
+shell_append=no
+( foo=bar; set foo baz; eval "$[1]+=\$[2]" && test "$foo" = barbaz ) \
+    >/dev/null 2>&1 \
+  && shell_append=yes
+AC_MSG_RESULT([$shell_append])
+if test "x$shell_append" = "xno"; then
+       AC_MSG_ERROR([Your shell (/bin/sh) is not supporting += which is required by this configure script. 
Try CONFIG_SHELL="env bash" ./configure instead.])
+fi
+
 # I18N
 IT_PROG_INTLTOOL([0.25])