browser-plugin: Add support for BROWSER_PLUGIN_DIR environment variable

This makes it easy to override where the plugin should be installed.

This is based on what totem does.

https://bugzilla.gnome.org/show_bug.cgi?id=659123

browser-plugin/Makefile.am

@@ -1,5 +1,5 @@
 
-mozillalibdir = $(libdir)/mozilla/plugins
+mozillalibdir = $(BROWSER_PLUGIN_DIR)
 
 mozillalib_LTLIBRARIES = libgnome-shell-browser-plugin.la
 

configure.ac

@@ -235,6 +235,9 @@ else
 fi
 AC_SUBST(SHELL_SYSTEM_CA_FILE,["$with_ca_certificates"])
 
+BROWSER_PLUGIN_DIR="${BROWSER_PLUGIN_DIR:-"\${libdir}/mozilla/plugins"}"
+AC_ARG_VAR([BROWSER_PLUGIN_DIR],[Where to install the plugin to])
+
 AC_CONFIG_FILES([
   Makefile
   data/Makefile