ScriptFu: build shared libscriptfu-3.0.so separate from executable script-fu

Changes are confined to the build system for ScriptFu, except one unnecessary include statement deleted from the source.

Why: so that other executables (future gimp-scheme-interpreter, or a future separated script-fu-server) can exist in separate directories, and share the library in memory (when built shared.)

Whether the library is built shared and installed on its own (versus static and not installed) is a compile time option (both automake LibTool and meson abstract it away) The default is shared and installed, say as libscriptfu-3.0.so.

A preliminary refactoring which helps enable MR !647 (closed)