Fix check_check for MinGW

[brarcher]

Currently the check_check unit tests do not run correctly in MinGW. When run on AppVeyor the following occurs:

if %platform%==mingw32 tests\check_check.exe
Running suite(s): Fork Sub
100%: Checks: 3, Failures: 0, Errors: 0
C:\projects\check\tests\check_check_fork.c:38:P:Core:test_inc:0: Passed
C:\projects\check\tests\check_check_fork.c:47:P:Core:test_nofork_sideeffects:0: Passed
C:\projects\check\tests\check_check_fork.c:54:P:Core:test_nofork_pid:0: Passed
Running suite(s): Fix Sub
0%: Checks: 1, Failures: 1, Errors: 0
C:\projects\check\tests\check_check_fixture.c:36:S:Fix Sub:unchecked_setup:0: Test failure in fixture
Running suite(s): Check Servant
Command exited with code -1073741819

[brarcher]

This was resolved in #102