Don't forbid shared library builds

[bretbrownjr]

Problem

Beman libraries currently hardcode STATIC in their add_library calls. This is:

• Practically not needed
• Limits the ability to build the projects as shard libraries, which standard C++ libraries do support
• Breaks support for the standard CMake feature to controlling how to choose between static or shared builds: BUILD_SHARED_LIBS

Proposal

In [CMAKE.LIBRARY_NAME], change this:

add_library(beman.smart_pointer STATIC)

To this:

add_library(beman.smart_pointer)

Further, require the latter style with justification as outlined in the "Problem" section of this issue.

Reference

For add_library supporting BUILD_SHARED_LIBS:
https://cmake.org/cmake/help/latest/command/add_library.html

Docs for BUILD_SHARED_LIBS, including that STATIC is the default for CMake built libraries.
https://cmake.org/cmake/help/latest/variable/BUILD_SHARED_LIBS.html#variable:BUILD_SHARED_LIBS

A PR to the Beman Exemplar to apply the relevant one-line change:
See bemanproject/exemplar#104.

[bretbrownjr]

Note that we could test different settings for BUILD_SHARED_LIBS, though I do get concerned about the combinatorial explosion of all the possible combinations of tests, both in terms of compute per commit pushed but also in terms of the amount of complexity in the GitHub actions configuration.