Fix (issue #205) inability to compile faust_butterfly C++ module when BUILD_MUTLITHREADING is OFF.

Add a header faust_openmp.h.in configured by cmake to define a compilation variable named OMP_ENABLED which is then used in faust_butterfly or faust_prod_opt modules to determine if OpenMP/C++ threads (posix threads) are available and enable a fallback code if they are not.