This allows building (and running) codec_unittest for android.
This explicitly needs to go at the end of the LDFLAGS, since both
this and -lgtest etc. are static libraries, and code in gtest
relies on the STL. Since this explicitly needs to be at the end,
a separate makefile variable is added for it.
The main purpose is to make sure that the build doesn't fail
if the build directory contains the gtest subdirectory, although
the unittests also could be run on a device.
One doesn't need to run the ndk-build command from within the
jni subdirectory, calling it from the project top level directory
works just as well.
Also remove an unnecessary step of cd ../../../.. at the end -
since all make commands are run in a subshell, we don't need to
restore the current working directory afterwards.
