Avoid building both a shared and static library

[JohannesKauffmann]

There is no reason to unconditionally build both variants. It increases
compile times, and consumers already know whether or not they want a
static or shared artifact. Instead, use a standard CMake construct to
build either one of them.

To allow users to override static vs shared for EIPScanner only, use a
new variable called EIPScanner_SHARED_LIBS: If set, our add_library()
will create a shared library, if not, a static library is created.
Building a static library by default also matches the default behaviour
of add_library().

This still allows users to specify BUILD_SHARED_LIBS on the command line
as an ultimate override, but at the same time avoids having
BUILD_SHARED_LIBS be the only option for building static vs shared.