scons detects standalone MSVC on Windows

Under Windows, Scons is now capable of detecting and compiling with
standalone MSVC compilers (aka "Visual C++ Build Tools").
http://landinghub.visualstudio.com/visual-cpp-build-tools

Tried with version 2015, and native x86 and x64 compilers under
Windows 10 pro 64 and Windows 8.1 64, with the default Win8 SDK
provided by the "Visual C++ Build Tools" web-installer.

Follow the same compiling instructions than for compiling with Visual
Studio, except that Visual Studio is no more required.

KNOWN ISSUES :
- ``methods.detect_visual_c_compiler_version()`` will emit a warning message
on computers where the ``VSINSTALLDIR`` environement variable is not present.
But it should compile just fine and still automatically detects the 32 or
64 bits according to the compiler you picked.

TODO :
- eventually, update ``platform/winrt/dectet.py`` with function
``methods.msvc_is_detected()`` and try to compile winrt/UWP with
these standalone compilers (if you did not select Win10 SDK when
installing the standalone tools, you can run it again).
- update doc to make users aware of "Visual C++ Build Tools" aka
"stadalone MSVC".
- eventually, update ``methods.detect_visual_c_compiler_version()``

methods.py

@@ -1516,6 +1516,12 @@ def detect_visual_c_compiler_version(tools_env):
 
         return vc_chosen_compiler_str
 
+def msvc_is_detected() :
+	# looks for VisualStudio env variable 
+	# or for Visual C++ Build Tools (which is a standalone MSVC)
+	return os.getenv("VSINSTALLDIR") or os.getenv("VS100COMNTOOLS") or os.getenv("VS110COMNTOOLS") or os.getenv("VS120COMNTOOLS") or os.getenv("VS140COMNTOOLS");
+
+
 def precious_program(env, program, sources, **args):
 	program = env.ProgramOriginal(program, sources, **args)
 	env.Precious(program)