build: figure out how to handle openmp

[fnrizzi]

No description provided.

[fnrizzi]

for example, use it by default if found:

find_package(OpenMP)

set(exename pdaWfExe)
add_executable(${exename} ${CMAKE_CURRENT_SOURCE_DIR}/cpp/main.cc)
target_compile_options(${exename} PRIVATE $<$<COMPILE_LANGUAGE:CXX>:-march=native>)
if(NOT OpenMP_CXX_FOUND)
  target_link_libraries(${exename} PRIVATE yaml-cpp)
else()
  target_link_libraries(${exename} PRIVATE yaml-cpp OpenMP::OpenMP_CXX)
  target_compile_definitions(${exename} PRIVATE PRESSIODEMOAPPS_ENABLE_OPENMP)
endif()