Patch cmake script for finding OpenMP on macOS/clang.

CMakeLists.txt

@@ -525,31 +525,37 @@ endif()
 
 
 if(BUILD_MULTITHREAD)
-	find_package(OpenMP REQUIRED)
-	if(NOT OpenMP_FOUND)
-		message(FATAL_ERROR "OpenMP not found.")
-	else()
-		message(STATUS "OpenMP is supported OpenMP_CXX_FOUND:  ${OpenMP_CXX_FOUND}, OpenMP_CXX_FLAGS: ${OpenMP_CXX_FLAGS}, OpenMP_CXX_LIB_NAMES: ${OpenMP_CXX_LIB_NAMES}, OpenMP_CXX_LIBRARY: ${OpenMP_CXX_LIBRARY}, OpenMP_CXX_LIBRARIES: ${OpenMP_CXX_LIBRARIES}")
-		foreach(OMP_FLAG IN LISTS OpenMP_CXX_FLAGS)
-			set(OMP_CXX_FLAGS "${OMP_CXX_FLAGS} ${OMP_FLAG}")
-		endforeach()
-		if(UNIX OR (WIN32 AND ${CMAKE_GENERATOR} MATCHES "MinGW Makefiles"))
-			foreach(OMP_LFLAG IN LISTS OpenMP_CXX_LIB_NAMES)
-				set(OMP_CXX_LFLAGS "${OMP_CXX_LFLAGS} -l${OMP_LFLAG}")
-			endforeach()
-			foreach(OMP_LFLAG IN LISTS OpenMP_CXX_LIBRARIES)
-				string(REGEX REPLACE "(.*)(/|\\\\).*" "\\1" OMP_LIB_PATH ${OMP_LFLAG})
-				set(OMP_CXX_LFLAGS " ${OMP_CXX_LFLAGS} -L${OMP_LIB_PATH}")
-			endforeach()
-		elseif(WIN32 AND ${CMAKE_GENERATOR} MATCHES "Visual Studio.*")
-			# strangely I noticed that findOpenMP gives -openmp instead of /openmp when compiler is msvc cl.exe
-			string(REGEX REPLACE "-" "/" OMP_CXX_FLAGS ${OMP_CXX_FLAGS})
+	find_package(OpenMP) #not REQUIRED, allow to fail the first time
+	if(NOT OpenMP_FOUND) # this section is particularly useful for clang/macOS configuration (OpenMP is not enabled by default, installed it via macports)
+		#  message(STATUS "OpenMP not found automatically, let's help it.")
+		set(OpenMP_CXX_FLAGS "-I${OpenMP_INC_DIR}")
+		set(OpenMP_CXX_LIB_NAMES "gomp")
+		# now retry
+		find_package(OpenMP REQUIRED) # last chance
+		if(NOT OpenMP_FOUND)
+			message(FATAL_ERROR "OpenMP not found. Please try to define cmake variables OpenMP_gomp_LIBRARY -- to the path of libgomp, lib included, and OpenMP_INC_DIR to the parent directory of omp.h")
 		endif()
-		string(REGEX REPLACE "(^[ \t\r\n]+)|([ \t\r\n]+$)" "" OMP_CXX_FLAGS ${OMP_CXX_FLAGS})
-		string(REGEX REPLACE "(^[ \t\r\n]+)|([ \t\r\n]+$)" "" OMP_CXX_LFLAGS ${OMP_CXX_LFLAGS})
-		message(STATUS OMP_CXX_FLAGS=${OMP_CXX_FLAGS} OMP_CXX_LFLAGS=${OMP_CXX_LFLAGS})
-		set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${OMP_CXX_FLAGS}")
 	endif()
+	message(STATUS "OpenMP is supported OpenMP_CXX_FOUND:  ${OpenMP_CXX_FOUND}, OpenMP_CXX_FLAGS: ${OpenMP_CXX_FLAGS}, OpenMP_CXX_LIB_NAMES: ${OpenMP_CXX_LIB_NAMES}, OpenMP_CXX_LIBRARY: ${OpenMP_CXX_LIBRARY}, OpenMP_CXX_LIBRARIES: ${OpenMP_CXX_LIBRARIES}")
+	foreach(OMP_FLAG IN LISTS OpenMP_CXX_FLAGS)
+		set(OMP_CXX_FLAGS "${OMP_CXX_FLAGS} ${OMP_FLAG}")
+	endforeach()
+	if(UNIX OR (WIN32 AND ${CMAKE_GENERATOR} MATCHES "MinGW Makefiles"))
+		foreach(OMP_LFLAG IN LISTS OpenMP_CXX_LIB_NAMES)
+			set(OMP_CXX_LFLAGS "${OMP_CXX_LFLAGS} -l${OMP_LFLAG}")
+		endforeach()
+		foreach(OMP_LFLAG IN LISTS OpenMP_CXX_LIBRARIES)
+			string(REGEX REPLACE "(.*)(/|\\\\).*" "\\1" OMP_LIB_PATH ${OMP_LFLAG})
+			set(OMP_CXX_LFLAGS " ${OMP_CXX_LFLAGS} -L${OMP_LIB_PATH}")
+		endforeach()
+	elseif(WIN32 AND ${CMAKE_GENERATOR} MATCHES "Visual Studio.*")
+		# strangely I noticed that findOpenMP gives -openmp instead of /openmp when compiler is msvc cl.exe
+		string(REGEX REPLACE "-" "/" OMP_CXX_FLAGS ${OMP_CXX_FLAGS})
+	endif()
+	string(REGEX REPLACE "(^[ \t\r\n]+)|([ \t\r\n]+$)" "" OMP_CXX_FLAGS ${OMP_CXX_FLAGS})
+	string(REGEX REPLACE "(^[ \t\r\n]+)|([ \t\r\n]+$)" "" OMP_CXX_LFLAGS ${OMP_CXX_LFLAGS})
+	message(STATUS OMP_CXX_FLAGS=${OMP_CXX_FLAGS} OMP_CXX_LFLAGS=${OMP_CXX_LFLAGS})
+	set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${OMP_CXX_FLAGS}")
 endif(BUILD_MULTITHREAD)
 
 #if( (CMAKE_BUILD_TYPE MATCHES "Debug") OR  (CMAKE_BUILD_TYPE MATCHES "debug") OR  (CMAKE_BUILD_TYPE MATCHES "DEBUG") )