`blt::openmp` INTERFACE_COMPILE_OPTIONS may not delimit lists correctly.

Nobody has claimed this yet.

`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'

```

Start in blt_add_target_compile_flags() and BLTSetupOpenMP.cmake, then reproduce the issue with the CMake project and OpenMP_ROOT=/opt/homebrew/opt/libomp shown in the report. Compare blt::openmp with OpenMP::OpenMP_CXX and verify that the compiler receives -Xclang and -fopenmp as separate arguments without the reported error.

Written by the indexing model from the issue text.