Export prebuilt tools to Make

Test: check out/soong/make_vars-aosp_arm.mk
Change-Id: I20bf062bbeedcc69dca9693a5c1bbd4c5fbfbe68
diff --git a/Android.bp b/Android.bp
index 43982e4..44d7092 100644
--- a/Android.bp
+++ b/Android.bp
@@ -11,6 +11,7 @@
 prebuilt_build_tool {
     name: "bison",
     deps: ["common/bison/**/*"],
+    export_to_make_var: "BISON",
     target: {
         darwin: {
             src: "darwin-x86/bin/bison",
@@ -25,6 +26,7 @@
 
 prebuilt_build_tool {
     name: "flex",
+    export_to_make_var: "LEX",
     target: {
         darwin: {
             src: "darwin-x86/bin/flex",
@@ -37,6 +39,7 @@
 
 prebuilt_build_tool {
     name: "m4",
+    export_to_make_var: "M4",
     target: {
         darwin: {
             src: "darwin-x86/bin/m4",