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>