Use extra_paths on Cygwin

Cygwin executables are still loaded by the Windows PE loader, so PATH needs
to include any extra directories where required DLLs can be found.

Cygwin uses a unix style ':'-separated PATH.  os.pathsep is used correctly
on extra_paths in meson_exe.py, but not in mesontest.py

mesonbuild/backend/backends.py

@@ -252,7 +252,7 @@ class Backend:
                 exe_wrapper = self.environment.cross_info.config['binaries'].get('exe_wrapper', None)
             else:
                 exe_wrapper = None
-            if mesonlib.is_windows():
+            if mesonlib.is_windows() or mesonlib.is_cygwin():
                 extra_paths = self.determine_windows_extra_paths(exe)
             else:
                 extra_paths = []
@@ -481,7 +481,7 @@ class Backend:
                 exe_wrapper = self.environment.cross_info.config['binaries'].get('exe_wrapper', None)
             else:
                 exe_wrapper = None
-            if mesonlib.is_windows():
+            if mesonlib.is_windows() or mesonlib.is_cygwin():
                 extra_paths = self.determine_windows_extra_paths(exe)
             else:
                 extra_paths = []

mesonbuild/backend/ninjabackend.py

@@ -492,7 +492,7 @@ int dummy;
         # the project, we need to set PATH so the DLLs are found. We use
         # a serialized executable wrapper for that and check if the
         # CustomTarget command needs extra paths first.
-        if target.capture or (mesonlib.is_windows() and
+        if target.capture or ((mesonlib.is_windows() or mesonlib.is_cygwin()) and
                               self.determine_windows_extra_paths(target.command[0])):
             exe_data = self.serialise_executable(target.command[0], cmd[1:],
                                                  # All targets are built from the build dir

mesontest.py

@@ -202,7 +202,7 @@ class TestHarness:
 
             child_env.update(test.env)
             if len(test.extra_paths) > 0:
-                child_env['PATH'] += ';'.join([''] + test.extra_paths)
+                child_env['PATH'] += os.pathsep.join([''] + test.extra_paths)
 
             # If MALLOC_PERTURB_ is not set, or if it is set to an empty value,
             # (i.e., the test or the environment don't explicitly set it), set