CMake: Pass OpenMP compiler and linker flags through CMake targets

[ywwry66]

Using OpenMP::OpenMP_LANG targets for CMake is less error-prone than passing the compiler and linker flags manually. Furthermore, it allows the user to customize those flags by setting OpenMP_LANG_FLAGS, OpenMP_LANG_LIB_NAMES, and OpenMP_omp_LIBRARY.

[ywwry66]

Ok, this is ready. If appropriate, I can also remove all OpenMP related code in cmake/fc.cmake to let CMake detect OpenMP Fortran flags automatically, as was done for C in 6aac065.

[dimpase]

see #5169 for the needed setup

[dimpase]

A successful test on Linux x86_64, where libomp is installed system-wide in /usr/. I ran the following:

cmake -B build -DUSE_OPENMP=1 -DBUILD_SHARED_LIBS=1 -DBUILD_STATIC_LIBS=0 -DOpenMP_C_FLAGS="-fopenmp"  -DOpenMP_C_LIB_NAMES="omp" -DOpenMP_Fortran_FLAGS="-fopenmp" -DOpenMP_Fortran_LIB_NAMES="omp" -DOpenMP_omp_LIBRARY="omp"
cd build/ && make -j8 && ctest

and the linkage is correct:

$ ldd lib/libopenblas.so
	linux-vdso.so.1 (0x00007f0cc7826000)
	libm.so.6 => /lib64/libm.so.6 (0x00007f0cc6141000)
	libomp.so => /usr/lib64/libomp.so (0x00007f0cc5ff8000)
	libc.so.6 => /lib64/libc.so.6 (0x00007f0cc5dfd000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f0cc7828000)

[dimpase]

Also works on M1 up to date system, with

cmake -B build -DUSE_OPENMP=1 -DBUILD_SHARED_LIBS=1 -DBUILD_STATIC_LIBS=0 -DOpenMP_C_FLAGS="-Xpreprocessor -fopenmp -I$(brew --prefix libomp)/include" -DOpenMP_C_LIB_NAMES="omp" -DOpenMP_Fortran_FLAGS="-fopenmp" -DOpenMP_Fortran_LIB_NAMES="omp" -DOpenMP_omp_LIBRARY=$(brew --prefix libomp)/lib/libomp.dylib

openblas.pc looks correct too.