Warn if shell isn't bash

We try to avoid bashism in build scripts but the claim
isn't rigorously validated on regular basis.  Warn
the possiblity that setting shell to bash (eg. from
dash in Ubuntu) might solve weird build problems.

Change-Id: I67790b3a632dd12c5e5ccd78d1ffe572679dd609
diff --git a/build/tools/prebuilt-common.sh b/build/tools/prebuilt-common.sh
index a83b92a..b452629 100644
--- a/build/tools/prebuilt-common.sh
+++ b/build/tools/prebuilt-common.sh
@@ -17,6 +17,11 @@
     fi
 fi
 
+# Warn about /bin/sh ins't bash.
+if [ -z "$BASH_VERSION" ] ; then
+    echo "WARNING: The shell running this script isn't bash.  Although we try to avoid bashism in scripts, things can happen."
+fi
+
 NDK_BUILDTOOLS_ABSPATH=$(cd $NDK_BUILDTOOLS_PATH && pwd)
 
 . $NDK_BUILDTOOLS_PATH/../core/ndk-common.sh