Bug 1209398 - Enable the FasterMake backend by default for desktop Firefox builds. r=gps

The order in which backends appear is important, and dealing with deduplication
in configure.in is not really nice, so for all simplification purposes, this relies
on using AC_SUBST_SET, which does the deduplication and keeps the original order
in which items appear (despite its name).

build/autoconf/config.status.m4

@@ -239,5 +239,5 @@ MOZ_ARG_ENABLE_STRING(build-backend,
                          Enable additional build backends],
 [ BUILD_BACKENDS="RecursiveMake `echo $enableval | sed 's/,/ /g'`"])
 
-AC_SUBST_LIST([BUILD_BACKENDS])
+AC_SUBST_SET([BUILD_BACKENDS])
 ])

configure.in

@@ -4031,6 +4031,8 @@ esac
 case "$MOZ_BUILD_APP" in
 browser)
   AC_DEFINE(MOZ_PHOENIX)
+
+  BUILD_BACKENDS="$BUILD_BACKENDS FasterMake"
   ;;
 
 xulrunner)