Add VPATH (src+obj for modules) and remove $(top_srcdir)/... to make
sure it's possible to do the build in a different directory and not
clutter the source with build results (such as Makefiles, *.o and *.a
files etc).

Closes: #1082
For VPATH builds:
* Add '../../spl' to spl source directory list to search
* Do a find in the found spl source for spl_config.h