SPERR does not build with OpenMP on macOS

[lindstro]

Describe the bug
Building SPERR with Apple clang on macOS results in this compiler warning and no OpenMP support:

clang: warning: argument unused during compilation: '-Xclang -fopenmp' [-Wunused-command-line-argument]

To Reproduce
Steps to reproduce the behavior:

1. mkdir build
2. cd build
3. cmake ..
4. make

Expected and actual behavior
The build works when commenting out this line in src/CMakeLists.txt:

  target_compile_options(   SPERR PUBLIC ${OpenMP_CXX_FLAGS} )

This line appears unnecessary for Linux builds.

Environment (please complete the following information):

• OS: macOS 11.7.2
• Compiler: AppleClang 13.0.0.13000029
• SPERR Version: 0.6 (and earlier versions)

[shaomeng]

Apple supplied toolchain has a weird relationship with OpenMP, but it does look like that CMakeLists.txt line doesn't help anything except than introducing a warning. I've removed that command; thanks for finding out the offending line Peter!