Cygwin: testsuite: Build testcases using automake

Cygwin: testsuite: Build testcases using automake

Build all the testcase executables directly using automake, rather than
passing the compiler information into DejaGnu to have it build them.

(This means you get build avoidance for these executables, so they only
get built once, rather than every time you run the test, and makes it
much easier to run them in isolatation against the installed Cygwin,
which is really nice to have when trying to fix broken tests...)

Rename the 'cygrun' subdirectory to 'mingw', and build all the testsuite
MinGW executables there.

Drop sample-miscompile.c (testing that compile failure is detected is
perhaps useful, but not here...)