build-sys: fix qemu-ga -pthread linking

When linking qemu-ga under some configuration (when gthread-2.0.pc
doesn't have -pthread, as happening atm with meson build), you may
have this linking issue:

/usr/bin/ld: libqemuutil.a(qemu-thread-posix.o): undefined reference to symbol 'pthread_setname_np@@GLIBC_2.12'
/usr/lib64/libpthread.so.0: error adding symbols: DSO missing from command line

Make sure qemu-ga links with the pthread library, by adding correct
flags to libs_qga.

This is really a QEMU bug, because it's QEMU code that's using pthread
functions, and so we must explicitly link against pthreads. The bug
was just masked by the fact that often some pkg-config or another for
one of our dependencies will add -pthread to the link line anyway.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20180104160523.22995-2-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>