Build script fixes:

- ensure all variables are defined, supports Mesa quick rebuilds;
- properly load VS2015 fallback environment with VS2017. It was working just because we were lucky.

buildscript/build.cmd

@@ -10,10 +10,10 @@
 @if %abi%==x64 set minabi=64
 @set vsenv=%ProgramFiles%
 @if NOT "%ProgramW6432%"=="" set vsenv=%vsenv% (x86)
-@set vsenv=%vsenv%\Microsoft Visual Studio\2017\Community\VC\Auxiliary\Build\vcvars%minabi%.bat
-@if EXIST "%vsenv%" @set toolchain=Visual Studio 15 2017
-@if NOT EXIST "%vsenv%" @set toolchain=Visual Studio 14
-@if NOT EXIST "%vsenv%" @set vsenv=%VS140COMNTOOLS%..\..\VC\bin\vcvars%minabi%.bat
+@set vsenv15=%vsenv%\Microsoft Visual Studio\2017\Community\VC\Auxiliary\Build\vcvars%minabi%.bat
+@if EXIST "%vsenv15%" @set toolchain=Visual Studio 15 2017
+@if NOT EXIST "%vsenv15%" @set toolchain=Visual Studio 14
+@set vsenv14=%VS140COMNTOOLS%..\..\VC\bin\vcvars%minabi%.bat
 @if %abi%==x64 @set toolchain=%toolchain% Win64
 @set gcc=%mesa%mingw-w64-%abi%\mingw%minabi%\bin
 @set vsenvloaded=0
@@ -27,6 +27,8 @@
 @RD /S /Q cmake-%abi%
 @md cmake-%abi%
 @cd cmake-%abi%
+@if EXIST "%vsenv15%" set vsenv=%vsenv15%
+@if NOT EXIST "%vsenv15%" set vsenv=%vsenv14%
 @set vsenvloaded=1
 @call "%vsenv%"
 @set PATH=%mesa%cmake\%abi%\bin\;%PATH%
@@ -59,16 +61,17 @@
 @cd "%mesa%mesa"
 @RD /S /Q build\windows-%longabi%
 @set /p openswr=Do you want to build OpenSWR drivers? (y=yes):
-@set buildswr=
-@if /i %openswr%==y @set buildswr=swr=1
+@set buildswr=0
+@if /i %openswr%==y @set buildswr=1
 @set mingw=n
 @set mingwtest=0
 @if EXIST "%gcc%" @set mingwtest=1
 @if %dxtnbuilt%==0 set PATH=%gcc%\;%PATH%
 @set msys2=%mesa%msys64\msys2_shell.cmd
 @if EXIST "%msys2%" @set mingwtest=%mingwtest%2
 @rem if %mingwtest%==12 @set /p mingw=Do you want to build with MinGW-W64 instead of Visual Studio (y=yes):
-@set buildmingw=
+@set mesatoolchain=default
+@set restartcond=0
 @set PATH=%mesa%Python\%abi%\;%mesa%Python\%abi%\Scripts\;%mesa%flexbison\;%mesa%m4\%abi%\usr\bin\;%PATH%
 @pip install -U mako
 @pip freeze > requirements.txt
@@ -77,7 +80,7 @@
 @if /i NOT %mingw%==y GOTO build_with_vs
 
 :build_with_mingw
-@set buildmingw=toolchain=crossmingw
+@set mesatoolchain=crossmingw
 @copy "%gcc%\%altabi%-w64-mingw32-gcc-ar.exe" "%gcc%\%altabi%-w64-mingw32-ar.exe"
 @copy "%gcc%\%altabi%-w64-mingw32-gcc-ranlib.exe" "%gcc%\%altabi%-w64-mingw32-ranlib.exe"
 @call "%msys2%" -use-full-path
@@ -91,12 +94,20 @@ pip freeze > requirements.txt
 pip install -r requirements.txt --upgrade
 cd $mesa
 cd mesa
-scons build=release platform=windows machine=x86 toolchain=crossmingw libgl-gdi
-@GOTO exit
+@GOTO build_mesa_exec
 
 :build_with_vs
-@if %vsenvloaded%==0 call "%vsenv%"
-@cmd /k "scons build=release platform=windows machine=%longabi% %buildmingw% %buildswr% libgl-gdi"
+@if %vsenvloaded%==0 call "%vsenv14%"
+@if %vsenvloaded%==1 @set restartcond=1
+@if EXIST "%vsenv15%" @set restartcond=%restartcond%2
+@if %restartcond%==12 (
+@echo "A Visual Studio build environment is already loaded. The script will now restart to flush loaded environment.
+@pause
+@GOTO exit
+)
+
+:build_mesa_exec
+@cmd /k "scons build=release platform=windows machine=%longabi% toolchain=%mesatoolchain% swr=%buildswr% libgl-gdi"
 
 :exit
 exit