blt_add_target_compile_flags() does not prefix SHELL: the way blt_add_target_link_flags() does. This means that BLTSetupOpenMP.cmake gets multiple arguments in the ${OpenMP_CXX_FLAGS} string, we get a single list element containing a space instead of a semicolon delimited list of arguments.

The OpenMP::OpenMP_CXX target deals with this by prefixing SHELL:, so that seems like a reasonable approach.

For a failure case, consider that the Apple Clang compilers with libomp from Homebrew require '-Xclang -fopenmp'. Configure the following CMake project with -DOpenMP_ROOT=/opt/homebrew/opt/libomp.

cmake_minimum_required(VERSION 3.15)
project(testbltopenmp)

set(ENABLE_OPENMP TRUE)

include(blt/SetupBLT.cmake)

blt_print_target_properties(TARGET blt::openmp)
blt_print_target_properties(TARGET OpenMP::OpenMP_CXX)

Result:

...

Problems were encountered while collecting compiler information:
	clang++: error: unknown argument: '-Xclang -fopenmp'