[rhythmbox] Enable configure to work on Dash in addition to Bash.

[Jonathan Matthew <jmatthew src gnome org>]

commit c9e6862c85d8b085af644e4f00f76b5732d0b992
Author: Ross Lagerwall <rosslagerwall gmail com>
Date:   Sat Apr 7 12:24:06 2012 +0200

    Enable configure to work on Dash in addition to Bash.

 configure.ac |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 33efca8..8ad4abd 100644
--- a/configure.ac
+++ b/configure.ac
@@ -712,7 +712,7 @@ AC_ARG_ENABLE([browser-plugin],
 	[AS_HELP_STRING([--enable-browser-plugin],[compile the iTunes detection browser plugin])],
 	[],[enable_browser_plugin=yes])
 
-MOZILLA_PLUGINDIR="${MOZILLA_PLUGINDIR:="\${libdir}/mozilla/plugins"}"
+MOZILLA_PLUGINDIR=${MOZILLA_PLUGINDIR:-'${libdir}/mozilla/plugins'}
 AC_ARG_VAR([MOZILLA_PLUGINDIR],[Where to install the browser plugin to])
 
 AM_CONDITIONAL([ENABLE_BROWSER_PLUGIN], test x$enable_browser_plugin = xyes)