Switch back to the old mechanism for API version.

It turns out that the API version was recently bumped in build/, but a
clean build of slang was needed to recognize it (actually just
slang_version.h).

Change-Id: I0e453c86bc700bb9659e23f37bde0d0c8a16f6b6
diff --git a/slang_version.h b/slang_version.h
index b73cae0..9ac9ca6 100644
--- a/slang_version.h
+++ b/slang_version.h
@@ -31,6 +31,7 @@
   SLANG_ICS_MR1_TARGET_API = 15,
   SLANG_JB_TARGET_API = 16,
   SLANG_JB_MR1_TARGET_API = 17,
+  SLANG_JB_MR2_TARGET_API = 18,
   SLANG_MAXIMUM_TARGET_API = RS_VERSION
 };
 // Note that RS_VERSION is defined at build time (see Android.mk for details).