appveyor: add Cygwin build

Closes #8693

appveyor.yml

@@ -243,6 +243,14 @@ environment:
         DISABLED_TESTS: "!19 !504 !704 !705 ~1056 !1233"
         ADD_PATH: "C:\\msys64\\usr\\bin"
         CONFIG_ARGS: "--enable-warnings --enable-werror --with-schannel"
+      # autotools-based Cygwin build
+      - APPVEYOR_BUILD_WORKER_IMAGE: "Visual Studio 2022"
+        BUILD_SYSTEM: autotools
+        TESTING: ON
+        DISABLED_TESTS: "~1056"
+        ADD_PATH: "C:\\cygwin64\\bin"
+        CONFIG_ARGS: "--enable-debug --enable-werror --disable-threaded-resolver --without-ssl"
+        POSIX_PATH_PREFIX: "/cygdrive"
 
 install:
     - if not "%ADD_PATH%"=="" (
@@ -288,7 +296,7 @@ build_script:
         ..\builds\libcurl-vc14.10-x64-%PATHPART%-dll-ssl-dll-ipv6-sspi\bin\curl.exe -V
       ) else (
       if %BUILD_SYSTEM%==autotools (
-        bash.exe -e -l -c "cd /c/projects/curl && autoreconf -fi && ./configure %CONFIG_ARGS% && make V=1 && make V=1 examples && cd tests && make V=1"
+        bash.exe -e -l -c "cd %POSIX_PATH_PREFIX%/c/projects/curl && autoreconf -fi && ./configure %CONFIG_ARGS% && make V=1 && make V=1 examples && cd tests && make V=1"
       )))))
     - if %TESTING%==ON (
         if %BUILD_SYSTEM%==CMake (
@@ -302,9 +310,9 @@ test_script:
           cmake --build . --config %PRJ_CFG% --target test-ci
         ) else (
         if %BUILD_SYSTEM%==autotools (
-          bash.exe -e -l -c "cd /c/projects/curl && make V=1 TFLAGS='%DISABLED_TESTS%' test-ci"
+          bash.exe -e -l -c "cd %POSIX_PATH_PREFIX%/c/projects/curl && make V=1 TFLAGS='%DISABLED_TESTS%' test-ci"
         ) else (
-          bash.exe -e -l -c "cd /c/projects/curl/tests && ./runtests.pl -a -p !flaky -r -rm %DISABLED_TESTS%"
+          bash.exe -e -l -c "cd %POSIX_PATH_PREFIX%/c/projects/curl/tests && ./runtests.pl -a -p !flaky -r -rm %DISABLED_TESTS%"
         ))
       )