fix typo in last commit
use multiprocessing to parallelize the test suite
Remove attempt to test system header from tests/plugin/callgraph

(The output varied too much between different compiler releases)
Fix selftests broken by d916a0f5edfb8639d491e232778fa5a04f0503ad
Add api-report Makefile target
gcc-c-api: add 'dereference' attribute to array_type, pointer_type, vector_type
Use gcc_ssa_name_get_version rather than SSA_NAME_VERSION in PyGccSsaName_repr
Make various things be METH_NOARGS