Support building internal libraries as shared. This reduces
development time by eliminating the need to re-link all
binaries every time non-interface code in the library changes.
Instead, can hack on libxyz, then `make libxyz`, and re-run
monerod.
By default BUILD_SHARED_LIBS is OFF in release build type,
and ON in debug build type, but can be overriden with -D.
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