CMake: add option to set BUILD_SHARED_LIBS variable

From https://cmake.org/cmake/help/latest/variable/BUILD_SHARED_LIBS.html

:

"Global flag to cause add_library() to create shared libraries if on.

If present and true, this will cause all libraries to be built shared unless the library was explicitly added as a static library. This variable is often added to projects as an option() so that each user of a project can decide if they want to build the project using shared or static libraries."

Signed-off-by: Jean-Marc Valin <jmvalin@jmvalin.ca>