[gnome-shell] build: Workaround quoting issues in configure script

[Bastien Nocera <hadess src gnome org>]

commit b05ab0eaebd9e6e58f1b9c092e05323c5b901a2b
Author: Alexander Tsoy <alexander tsoy me>
Date:   Mon Oct 27 16:45:10 2014 +0300

    build: Workaround quoting issues in configure script
    
    Default value of BROWSER_PLUGIN_DIR variable contains special symbols.
    Thus quoting and inlining it in parameter expansion is not portable.
    In particular it does not work in dash. Replace ${a:-b} parameter
    expansion with conditional statement.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=739241

 configure.ac |    4 +++-
 1 files changed, 3 insertions(+), 1 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index d55848b..5b8c306 100644
--- a/configure.ac
+++ b/configure.ac
@@ -232,7 +232,9 @@ esac
 AM_CFLAGS="$AM_CFLAGS $WARN_CFLAGS"
 AC_SUBST(AM_CFLAGS)
 
-BROWSER_PLUGIN_DIR="${BROWSER_PLUGIN_DIR:-"\${libdir}/mozilla/plugins"}"
+if test -z "${BROWSER_PLUGIN_DIR}"; then
+  BROWSER_PLUGIN_DIR="\${libdir}/mozilla/plugins"
+fi
 AC_ARG_VAR([BROWSER_PLUGIN_DIR],[Where to install the plugin to])
 
 AC_CONFIG_FILES([