configure: remove redundant AC_PATH_PROG(CYTHON,cython)
configure.in: find path for protoc and protoc-c, and subsitute into Makfile.in

Fail miserably if protoc not found.
Makefile.inc.in: substitutions for PROTOC and PROTOC_C
machinetalk/Submakefile: use substitutions for PROTOC and PROTOC_C
machinetalk/Submakefile: skip superfluous install targets