By convention projects using CMake which can build either static or
shared libraries use a BUILD_SHARED_LIBS flag to allow selecting between
both: the add_library() command automatically switches between both using
this variable when the library kind is not passed to add_library(). It
is also usual to expose the BUILD_SHARED_LIBS as an user-facing setting
with the option() command.
This way, the following will both work as expected:
% cmake -DBUILD_SHARED_LIBS=OFF ...
% cmake -DBUILS_SHARED_LIBS=ON ...
This is helpful for distributions which need (or want) to build only
static libraries.
