This adds a suite of unit tests for libsigrok. It uses the 'Check'
tool/library (apt-get install check) to run the tests.

The configure tool tries to find libcheck. If it succeeds, a
"make check" will run all tests. Otherwise, none of the tests will
be built and "make check" will not run any tests.

This also means that users who don't have 'check' installed will still
be able to build and install libsigrok just fine.