[PATCH] Add minimal CMake config and improve windows MSVC support

Add support for MSVC compiler. This involved fixing several compatibility
issue and updating jitterentroipy.h to pick the right platform specific
header.

Setup:

```
cd ~/
git clone git@github.com:smuellerDD/jitterentropy-library.git
mkdir jitter-build
cd jitter-build
```

Quickstart for new CMake build on LInux with Make

```
cmake ../jitterentropy-library
make
```

To use Ninja:

```
cmake -G Ninja ../jitterentropy-library
ninja
```

To use Ninja with AWS-LC:

```
cd ~/
git clone https://github.com/awslabs/aws-lc.git


mkdir aws-lc-build aws-lc-install
cd aws-lc-build
cmake -G Ninja -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=~/aws-lc-install ../aws-lc
ninja install
cd ~/jitter-build
cmake -G Ninja -DEXTERNAL_CRYPTO=AWSLC -DCMAKE_PREFIX_PATH=~/aws-lc-install ../jitterentropy-library
```

Note: We install AWS-LC into the path defined with CMAKE_INSTALL_PREFIX
and tell Jitter to use it by defining CMAKE_PREFIX_PATH.

Signed-off-by: Andrew Hopkins <andhop@amazon.com>
Signed-off-by: Stephan Mueller <smueller@chronox.de>