Run internal tests in Cygwin

[Rangi42]

Half of #1525

Based on:

• https://github.com/curl/curl/blob/master/.github/workflows/windows.yml
• https://github.com/pypa/distutils/blob/main/.github/workflows/main.yml

[Rangi42]

We probably don't need to test both 32- and 64-bit Cygwin, but I'm not sure which one we should pick.

[Rangi42]

run-tests.sh: internal error

Hmm. I'm not sure verified Cygwin support is worth continuing to debug this.

[Rangi42]

Progress! RGBDS builds, and tests run up until it tries to build rgbgfx_test:

test/gfx/rgbgfx_test.cpp:311:9: error: 'siginfo_t' was not declared in this scope
  311 |         siginfo_t info;
      |         ^~~~~~~~~
test/gfx/rgbgfx_test.cpp:312:20: error: 'P_PID' was not declared in this scope
  312 |         if (waitid(P_PID, pid, &info, WEXITED) != 0) {
      |                    ^~~~~
test/gfx/rgbgfx_test.cpp:312:33: error: 'info' was not declared in this scope
  312 |         if (waitid(P_PID, pid, &info, WEXITED) != 0) {
      |                                 ^~~~
test/gfx/rgbgfx_test.cpp:312:39: error: 'WEXITED' was not declared in this scope; did you mean 'WIFEXITED'?
  312 |         if (waitid(P_PID, pid, &info, WEXITED) != 0) {
      |                                       ^~~~~~~
      |                                       WIFEXITED
test/gfx/rgbgfx_test.cpp:312:13: error: 'waitid' was not declared in this scope; did you mean 'waitpid'?
  312 |         if (waitid(P_PID, pid, &info, WEXITED) != 0) {
      |             ^~~~~~
      |             waitpid
test/gfx/rgbgfx_test.cpp:319:21: error: 'strsignal' was not declared in this scope; did you mean 'signal'?
  319 |                     strsignal(info.si_status),
      |                     ^~~~~~~~~
      |                     signal

Edit: Looks like Cygwin needs __POSIX_VISIBLE >= 199309.

Making sure _POXIC_C_SOURCE is defined fixed that, but there are still these left:

test/gfx/rgbgfx_test.cpp:318:20: error: 'P_PID' was not declared in this scope
  318 |         if (waitid(P_PID, pid, &info, WEXITED) != 0) {
      |                    ^~~~~
test/gfx/rgbgfx_test.cpp:318:39: error: 'WEXITED' was not declared in this scope; did you mean 'WIFEXITED'?
  318 |         if (waitid(P_PID, pid, &info, WEXITED) != 0) {
      |                                       ^~~~~~~
      |                                       WIFEXITED
test/gfx/rgbgfx_test.cpp:318:13: error: 'waitid' was not declared in this scope; did you mean 'waitpid'?
  318 |         if (waitid(P_PID, pid, &info, WEXITED) != 0) {
      |             ^~~~~~
      |             waitpid

Edit: Using waitpid avoids the need for siginfo_t and P_PID entirely.