cmake build assumes static boost libraries

[MarkusTeufelberger]

In https://github.com/ripple/rippled/blob/develop/Builds/CMake/CMakeFuncs.cmake#L340 set(Boost_USE_STATIC_LIBS on) is always true instead of being on a switch via if (static)

Static libraries are not always shipped, especially for boost. Please reconsider linking boost dynamically and leaving the option of linking it statically up to the user. There is already a build time option to build a static binary via cmake. https://github.com/ripple/rippled/blob/develop/Builds/CMake/CMakeFuncs.cmake#L171 seems to support this.