Only export restbed symbols when building as a shared library + fix MinGW build

[madebr]

Fixes #531 by:

• only exporting the restbed symbols when building restbed as a shared library.
  On Windows, when linking to a static restbed, you now need to add -DRESTBED_STATIC to the compiler.
• When installing, also install the binary component of a shared library. This is the .dll on Windows.
• Fix issues when building restbed using MinGW@Linux. In addition to gcc@Linux, I used this toolchain to verify the correct symbol visibility of restbed's users.

Fixes #496 too