appveyor: Revert commits adding Cygwin support.

This reverts commits 00ad77b9f683e561b1ac45fbb89eb2bafe45c8c6 and
5334dafee265d78abdfcf30e2c693e0791bfecf5.

This avoids Appveyor build breakage due to Cygwin, but more importantly,
there are several problems with these patches, as highlighted to my
recent mesa-dev mail.  So better to revert for now, and pursue Cygwin
support after these have been address.
diff --git a/appveyor.yml b/appveyor.yml
index 9c6e5ac..ccb84fd 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -38,9 +38,6 @@
 - '%LOCALAPPDATA%\pip\Cache -> appveyor.yml'
 - win_flex_bison-2.5.15.zip
 - llvm-5.0.1-msvc2017-mtd.7z
-- C:\pkgcache
-- 'C:\cygwin64\home\%USERNAME%\.ccache'
-- 'C:\cygwin\home\%USERNAME%\.ccache'
 
 os: Visual Studio 2017
 
@@ -50,23 +47,43 @@
 - git config --global core.autocrlf true
 
 environment:
-  matrix:
-  - compiler: msvc
-    WINFLEXBISON_VERSION: 2.5.15
-    LLVM_ARCHIVE: llvm-5.0.1-msvc2017-mtd.7z
-  - compiler: cygwin
-    arch: x64
+  WINFLEXBISON_VERSION: 2.5.15
+  LLVM_ARCHIVE: llvm-5.0.1-msvc2017-mtd.7z
 
 install:
-- call scripts\appveyor_%compiler%.bat install
+# Check git config
+- git config core.autocrlf
+# Check pip
+- python --version
+- python -m pip --version
+# Install Mako
+- python -m pip install Mako==1.0.7
+# Install pywin32 extensions, needed by SCons
+- python -m pip install pypiwin32
+# Install python wheels, necessary to install SCons via pip
+- python -m pip install wheel
+# Install SCons
+- python -m pip install scons==3.0.1
+- scons --version
+# Install flex/bison
+- set WINFLEXBISON_ARCHIVE=win_flex_bison-%WINFLEXBISON_VERSION%.zip
+- if not exist "%WINFLEXBISON_ARCHIVE%" appveyor DownloadFile "https://github.com/lexxmark/winflexbison/releases/download/v%WINFLEXBISON_VERSION%/%WINFLEXBISON_ARCHIVE%"
+- 7z x -y -owinflexbison\ "%WINFLEXBISON_ARCHIVE%" > nul
+- set Path=%CD%\winflexbison;%Path%
+- win_flex --version
+- win_bison --version
+# Download and extract LLVM
+- if not exist "%LLVM_ARCHIVE%" appveyor DownloadFile "https://people.freedesktop.org/~jrfonseca/llvm/%LLVM_ARCHIVE%"
+- 7z x -y "%LLVM_ARCHIVE%" > nul
+- mkdir llvm\bin
+- set LLVM=%CD%\llvm
 
 build_script:
-- call scripts\appveyor_%compiler%.bat build_script
+- scons -j%NUMBER_OF_PROCESSORS% MSVC_VERSION=14.1 llvm=1
 
 after_build:
-- call scripts\appveyor_%compiler%.bat after_build
+- scons -j%NUMBER_OF_PROCESSORS% MSVC_VERSION=14.1 llvm=1 check
 
-test: off
 
 # It's possible to setup notification here, as described in
 # http://www.appveyor.com/docs/notifications#appveyor-yml-configuration , but