Update configure.py to support set CXX to LLVM clang-cl for MSVC platform.

[zufuliu]

This PR also disabled generating configure task for Windows when any configuration variable is set, to fix CreateProcess failure:

H:\llvm\ninja-fork>ninja.bootstrap.exe configure
[1/1] CXX=clang-cl python3.exe ./configure.py
FAILED: [code=1] build.ninja
CXX=clang-cl python3.exe ./configure.py
CreateProcess failed: The system cannot find the file specified.
ninja: error: rebuilding 'build.ninja': subcommand failed

-if not host.is_mingw():
+if not host.is_windows() or not configure_env:

ninja/configure.py

Lines 748 to 757 in 6b5d7a7

	if not host.is_mingw():
	n.comment('Regenerate build files if build script changes.')
	n.rule('configure',
	command='${configure_env}%s $root/configure.py $configure_args' %
	options.with_python,
	generator=True)
	n.build('build.ninja', 'configure',
	implicit=['$root/configure.py',
	os.path.normpath('$root/misc/ninja_syntax.py')])
	n.newline()

[jhasse]

Not sure if we should reimplement CMake in configure.py ...

[zufuliu]

Currently configure.py set more options than CMakeLists.txt, though some of them (mostly /wd or -Wno) may be irrelevant now. run configure.py --bootstrap is lightweight than run cmake -G "Ninja" -DCMAKE_BUILD_TYPE=Release ../ninja.