CMake: add BUILD_SHARED_LIBS option

Prior to commit 4e955731, ngtcp2 was
built as shared library. That has changed into a static or shared
library depending on the BUILD_SHARED_LIBS option, but this option was
not documented and the comments were misleading. Rectify both issues.

Note that autotools builds both libraries by default, but the current
CMake config is only able to build one of them.