Link codec_unittest to the static android STL

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.
Merge pull request #350 from mstorsjo/android-unittest

Link codec_unittest to the static android STL