In MacOS, gcc and g++ are simlink to Apple Clang which does not have OpenMP. We had to manually install and then compile.
We followed steps from this reference: fritzsedlazeck/Sniffles#136 (comment)
First, Installed zlib, cmake >= 2.8, gzip and bzip2.
Then, brew install gcc. It will create gcc-ver and g++-ver with a version number in /usr/local/bin so that they do not conflict with MacOS default gcc/g++. The two new versioned gcc/g++ have OpemMP by default. Finally we can compile MEGAHIT by telling cmake where the real gcc and g++ are, using -D CMAKE_C_COMPILER=/usr/local/bin/gcc-ver -D CMAKE_CXX_COMPILER=/usr/local/bin/g++-ver (replace the ver with real installed version number).
We used the following commands to compile MEGAHIT.
git clone https://github.com/voutcn/megahit.git
cd megahit
git submodule update --init
mkdir build && cd build
cmake .. -D CMAKE_C_COMPILER=/usr/local/bin/gcc-14 -D CMAKE_CXX_COMPILER=/usr/local/bin/g++-14 -D CMAKE_BUILD_TYPE=Release
make -j4