appveyor: build with CMake under Cygwin

appveyor.yml

@@ -187,6 +187,9 @@
   version: 1.0.{build}
   environment:
     matrix:
+    - COMPILER: "gcc"
+      HOST:     "cygwin"
+      PLATFORM: "x64"
     - COMPILER: "gcc"
       HOST:     "mingw"
       PLATFORM: "x64"
@@ -220,6 +223,14 @@
   install:
   - ECHO Installing %COMPILER% %PLATFORM% %CONFIGURATION%
   - SET PATH_ORIGINAL=%PATH%
+  - if [%HOST%]==[cygwin] (
+      ECHO Installing Cygwin Packages &&
+      C:\cygwin64\setup-x86_64.exe -qnNdO -R "C:\cygwin64" -g -P ^
+        gcc-g++,^
+        gcc,^
+        cmake,^
+        make
+    )
   - if [%HOST%]==[mingw] (
       SET "PATH_MINGW32=C:\mingw-w64\i686-6.3.0-posix-dwarf-rt_v5-rev1\mingw32\bin" &&
       SET "PATH_MINGW64=C:\mingw-w64\x86_64-6.3.0-posix-seh-rt_v5-rev1\mingw64\bin" &&
@@ -232,6 +243,16 @@
 
   build_script:
   - ECHO Building %COMPILER% %PLATFORM% %CONFIGURATION%
+  - if [%HOST%]==[cygwin] (
+      set CHERE_INVOKING=yes &&
+      set CC=%COMPILER% &&
+      C:\cygwin64\bin\bash --login -c "
+        set -e;
+        cd build/cmake;
+        CFLAGS='-Werror' cmake -G 'Unix Makefiles' .;
+        make -j4;
+      "
+    )
   - if [%HOST%]==[mingw] (
       ( if [%PLATFORM%]==[x64] (
         SET "PATH=%PATH_MINGW64%;%PATH_ORIGINAL%"