The install target is not configurable

[dvzrv]

Hi! 👋

When upgrading the Arch Linux package of reapack to 1.2.4.5 I noticed we are manually copying the built shared object file to a target location.

This seems to be because the target location is not configurable:

reapack/CMakeLists.txt

Lines 123 to 128 in 1727aa3

	set(REAPER_USER_PLUGINS "UserPlugins")
	install(TARGETS reaper_reapack
	COMPONENT ReaPack
	RUNTIME DESTINATION "${REAPER_USER_PLUGINS}" # Windows .dll
	LIBRARY DESTINATION "${REAPER_USER_PLUGINS}" # Linux .so/macOS .dylib
	)

Could this be adjusted, so that it becomes configurable? (related to #80)