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.

CMakeLists.txt

@@ -549,7 +549,9 @@ if(BUILD_MULTITHREAD)
 	endif()
 	message(STATUS OMP_CXX_FLAGS=${OMP_CXX_FLAGS} OMP_CXX_LFLAGS=${OMP_CXX_LFLAGS})
 	set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${OMP_CXX_FLAGS}")
+	set(OMP_ENABLED ON) # (CPP preprocessor compilation constant) used in cpp code to detect if OMP is usable
 endif(BUILD_MULTITHREAD)
+configure_file(${FAUST_SRC_DIR}/utils/faust_openmp.h.in ${FAUST_SRC_DIR}/utils/faust_openmp.h)
 
 #if( (CMAKE_BUILD_TYPE MATCHES "Debug") OR  (CMAKE_BUILD_TYPE MATCHES "debug") OR  (CMAKE_BUILD_TYPE MATCHES "DEBUG") )
 if (BUILD_DEBUG OR (CMAKE_BUILD_TYPE MATCHES "Debug") OR  (CMAKE_BUILD_TYPE MATCHES "debug") OR  (CMAKE_BUILD_TYPE MATCHES "DEBUG") )

src/algorithm/factorization/faust_butterfly.hpp

 #include "faust_MatDense.h"
 #include "faust_TransformHelper.h"
 #include <vector>
-#include "omp.h" //TODO: ifdef to avoid when BUILD_MULTITHREAD is OFF
+#include "faust_openmp.h"
+#ifdef OMP_ENABLED
+#include "omp.h"
+#endif
 #include <cmath>
 #include <limits>
 
@@ -40,7 +43,11 @@ namespace Faust
 			//	Vect<FPP, Cpu> c1(r), c2(r), r1(r), r2(r);
 			vector<bool> processed_ids(r, false);
 			auto eps = 1e-6;
+#ifdef OMP_ENABLED
 			int nthreads = 8;
+#else
+			int nthreads = 1;
+#endif
 			auto th_class = new vector<faust_unsigned_int>[nthreads];
 			for(int i=0;i<r;i++)
 			{
@@ -56,7 +63,11 @@ namespace Faust
 						{
 							if(s1.eq_cols(s2, i, j, eps) && s1.eq_rows(s2, i, j, eps))
 							{
+#ifdef OMP_ENABLED
 								th_class[omp_get_thread_num()].push_back(j);
+#else
+								th_class[0].push_back(j);
+#endif
 								processed_ids[j] = true;
 							}
 						}

src/faust_linear_operator/CPU/faust_prod_opt.hpp

+#include "faust_openmp.h"
 //TODO: allow nflags == 1 and all factors using with same flag
 /**
  *	\brief Multiply all the matrices together (as a product of n factors) with an optimization based on the associativity of the matrix product ; following an order that minimizes the number of scalar operations.
@@ -346,7 +347,7 @@ template<typename FPP, FDevice DEVICE>
 Faust::MatDense<FPP,DEVICE> Faust::multiply_omp(const std::vector<Faust::MatGeneric<FPP,DEVICE>*> data, const Faust::MatDense<FPP,DEVICE> A, const char opThis)
 {
 	Faust::MatDense<FPP, DEVICE> *M = nullptr;
-#ifdef _MUL_OMP_ //TODO: this compiler constant should be defined auto. when BUILD_MULTITHREADING is ON
+#ifdef OMP_ENABLED
 				// until this this method is disabled at compilation unless we manually define the constant in CFLAGS (for example).
 	int nth, start_nth, thid, num_per_th, data_size;
 	Faust::MatDense<FPP,DEVICE>* mats[8];
@@ -445,6 +446,8 @@ Faust::MatDense<FPP,DEVICE> Faust::multiply_omp(const std::vector<Faust::MatGene
 template<typename FPP, FDevice DEVICE>
 Faust::MatDense<FPP,DEVICE> Faust::multiply_par(const std::vector<Faust::MatGeneric<FPP,DEVICE>*>& data, const Faust::MatDense<FPP,DEVICE> A, const char opThis)
 {
+
+#ifdef OMP_ENABLED // this function doesn't use OpenMP but C++ threads are implemented using POSIX threads on Linux gcc as OpenMP, so for FAµST we assume it's the same
 	int nth = std::thread::hardware_concurrency(); // https://en.cppreference.com/w/cpp/thread/thread/hardware_concurrency
 	int barrier_count = nth;
 	Faust::MatDense<FPP, DEVICE> *M;
@@ -504,7 +507,9 @@ Faust::MatDense<FPP,DEVICE> Faust::multiply_par(const std::vector<Faust::MatGene
 	//TODO: return a ptr instead of a copy
 	//TODO: delete threads
 	return *M;
-
+#else
+	throw std::runtime_error("It's not possible to call Faust::multiply_par because the library hasn't been compiled with this function enabled.");
+#endif
 }
 
 template<typename FPP, FDevice DEVICE>

src/utils/faust_openmp.h.in

+#cmakedefine OMP_ENABLED // #define OMP_ENABLED if BUILD_MULTITHREAD=ON, not defined otherwise (see the main CMakeLists.txt for the variable def)
+