No DLL export/import macros nor usage of such

[louis-langholtz]

Expected/Desired Behavior or Experience:

PlayRho fully supports being used as a dynamic link library (DLL). There are DLL export/import macros defined in a common include file and these are used as necessary.

Actual Behavior:

While there should be nothing stopping PlayRho from being used as a dynamic link library, there's also currently no macros defined for that nor usage of such.

[louis-langholtz]

Also, consider using an inline ABI namespace along with this change.

[louis-langholtz]

This has been overcome by events (OBE). PlayRho, at least from the master branch, fully supports being used as a DLL for all tested platforms. If that's not already the case in an existing release, release 2.0 will get this.