Add libraries such as -lm and -lpthread to the tests build line.
Add libraries such as -lm and -lpthread to the tests build line.

This prevents a compile error on Debian Jessie, from git, such as this:
/usr/bin/ld: test_playback.o: undefined reference to symbol 'sin@@GLIBC_2.2.5'
//lib/x86_64-linux-gnu/libm.so.6: error adding symbols: DSO missing from command line

This is fairly subtle, and Debian specific.  It only happens when you use
autoreconf to generate a new libtool script.  Debian patches that script
to require an explicit setting to link with all dependent libraries.

It should be harmless on other distros, and it does save us Debian guys some
hassle.

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