fix deciding which externals directories to build

The way it was doing it before was completely brain-dead.

Change-Id: Id1baac54b92f0cb4e68550fa135398bea02b060f
diff --git a/aos/build/externals.gyp b/aos/build/externals.gyp
index 44ca1d7..a097fd7 100644
--- a/aos/build/externals.gyp
+++ b/aos/build/externals.gyp
@@ -4,8 +4,8 @@
   'variables': {
     'externals': '<(AOS)/../output/downloaded',
     'externals_abs': '<!(readlink -f ../../output/downloaded)',
-    'compiled': '<(externals)/../compiled-<(ARCHITECTURE)<(SANITIZER_FPIE)',
-    'compiled_abs': '<(externals_abs)/../compiled-<(ARCHITECTURE)<(SANITIZER_FPIE)',
+    'compiled': '<(externals)/../compiled-<(ARCHITECTURE)<(EXTERNALS_EXTRA)',
+    'compiled_abs': '<(externals_abs)/../compiled-<(ARCHITECTURE)<(EXTERNALS_EXTRA)',
 
 # These versions have to be kept in sync with the ones in download_externals.sh.
     'eigen_version': '3.2.1',