Allows building simple targets with sane default flags.

For example, using the Android arm64 toolchain from the NDK:
https://developer.android.com/ndk/guides/standalone_toolchain.html
./build/tools/make-standalone-toolchain.sh --arch=arm64 \
  --platform=android-24 --install-dir=/tmp/arm64
CROSS=/tmp/arm64/bin/aarch64-linux-android- \
  ~/libvpx/configure --target=arm64-linux-gcc --disable-multithread

BUG=webm:1143

Change-Id: I06f5a7564f5382cf1a4bad41aef4308566c53adf