added linker option -lpthread if pthread.h is found
* include/sane/config.in.h: added HAVE_PTHREAD_H
* sanei/sanei_thread.c include/sane/sanei_thread.h: added pthread_
functions to library, so every backend is able to use either fork or
pthread for its reader-process - see plustek backend for example
* backend/plustek.h backend/plustek.c: added sanei_thread stuff to support
threading on MacOS X
* backend/plustek_pp*: some cleanup