try to fix cmake for OSX

@jeizenga, is this right?

CMakeLists.txt

@@ -8,23 +8,34 @@ project(odgi)
 set(CMAKE_CXX_STANDARD 14)
 
 # Use all standard-compliant optimizations
-set (CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -O3 -g")
-set (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -O3 -g")
-
-# Use openmp for parallelism, but it's configured differently on OSX
-find_package(OpenMP)
-if (OPENMP_FOUND)
-  if (${CMAKE_SYSTEM_NAME} MATCHES "Linux")
-    set (CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${OpenMP_C_FLAGS} -fopenmp")
-    set (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${OpenMP_CXX_FLAGS} -fopenmp")
-    set (CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} ${OpenMP_EXE_LINKER_FLAGS}")
-  elseif (${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
-    # assumes clang build
-    # we can't reliably detect when we're using clang, so for the time being we assume
-    set (CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${OpenMP_C_FLAGS} -Xpreprocessor -fopenmp")
-    set (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${OpenMP_CXX_FLAGS} -Xpreprocessor -fopenmp")
-    set (CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} ${OpenMP_EXE_LINKER_FLAGS} -lomp")
-  endif()
+set (CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${OpenMP_C_FLAGS} -O3 -mcx16 -g")
+set (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${OpenMP_CXX_FLAGS} -O3 -mcx16 -g")
+
+if (${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
+  # assumes clang build
+  # we can't reliably detect when we're using clang, so for the time being we assume
+  # TODO: can't we though?
+  # adapted from https://stackoverflow.com/questions/46414660/macos-cmake-and-openmp
+  # find_package(OpenMP) does not work reliably on macOS, so we do its work ourselves
+  set (OpenMP_C "${CMAKE_C_COMPILER}")
+  set (OpenMP_C_FLAGS " -Xpreprocessor -fopenmp -I/opt/local/include/libomp -I/usr/local/include -L/opt/local/lib/libomp -L/usr/local/lib")
+  set (OpenMP_C_LIB_NAMES "libomp" "libgomp" "libiomp5")
+  set (OpenMP_CXX "${CMAKE_CXX_COMPILER}")
+  set (OpenMP_CXX_FLAGS " -Xpreprocessor -fopenmp -I/opt/local/include/libomp -I/usr/local/include -L/opt/local/lib/libomp -L/usr/local/lib")
+  set (OpenMP_CXX_LIB_NAMES "libomp" "libgomp" "libiomp5")
+  set (OpenMP_libomp_LIBRARY "omp")
+  set (OpenMP_libgomp_LIBRARY "gomp")
+  set (OpenMP_libiomp5_LIBRARY "iomp5")
+  # and now add the OpenMP parameters to the compile flags
+  set (CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${OpenMP_C_FLAGS}")
+  set (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${OpenMP_CXX_FLAGS}")
+  set (CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} ${OpenMP_EXE_LINKER_FLAGS} -lomp")
+elseif (${CMAKE_SYSTEM_NAME} MATCHES "Linux")
+  find_package(OpenMP REQUIRED)
+  # add the flags it detects to the compile flags
+  set (CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${OpenMP_C_FLAGS} -fopenmp")
+  set (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${OpenMP_CXX_FLAGS} -fopenmp")
+  set (CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} ${OpenMP_EXE_LINKER_FLAGS}")
 endif()
 
 # Set the output folder where your program will be created