bug 610938 - build bustage when configuring with relative path, can't find ads2gas.pl r=khuey a=gavin

configure.in

@@ -6105,7 +6105,7 @@ if test -n "$MOZ_WEBM" -a -z "$MOZ_NATIVE_LIBVPX"; then
         dnl opcodes; actual arch detection is done at runtime.
         VPX_ASFLAGS="-march=armv7-a -mfpu=neon"
         VPX_DASH_C_FLAG="-c"
-        VPX_AS_CONVERSION="$PERL ${srcdir}/media/libvpx/build/make/ads2gas.pl"
+        VPX_AS_CONVERSION='$(PERL) $(topsrcdir)/media/libvpx/build/make/ads2gas.pl'
         VPX_ASM_SUFFIX="$ASM_SUFFIX"
         VPX_ARM_ASM=1
       fi