Change finufft targets to provide proper install interface include dirs

[Wentzell]

Change finufft targets to provide proper install interface include dirs

[ahbarnett]

Since I still don't use cmake much, I don't understand what's changing here. Please can you explain a little? Thanks! Alex

[Wentzell]

This commit changes the include directory location for the library targets such as finufft and finufft_static depending on the context in which they are used. For the finufft build itself, i.e. before installing the library, the ${CMAKE_CURRENT_SOURCE_DIR}/include directory should be used. For other projects consuming an installed version of FINUFFT, the ${CMAKE_INSTALL_PREFIX}/include directory should be used.