improper Run-Time Library selection for ICC and MSVC Static builds on Windows

Hello, everyone,

For BOOST builds using ICC and MSVC with Static layout got warnings:
```
cl : Command line warning D9025 : overriding '/MDd' with '/MTd'
```
for Debug configuration, and
```
cl : Command line warning D9025 : overriding '/MD' with '/MT'
```
for Release configuration.

Reproduced, if `/MTd` and `/MT` keys are explicitly set using `cxxflags` variable.

The possible workaround is to remove `/MDd` and `/MD` keys from [msvc.jam](https://github.com/boostorg/build/blob/develop/src/tools/msvc.jam) before build and select Run-Time Library using `cxxflags` variable. But it would be much convenient, if appropriate Run-Time Libraries were selected according its [description](https://msdn.microsoft.com/en-us/library/2kzt1wy3.aspx) by default. If there is no reason to do otherwise, of course.

Environment:
- Windows 10 x64,
- Intel Parallel Studio XE 2017 Update 4,
- Visual Studio 2015 Update 3,
- BOOST-1.64.0.

Best,

Alexander