make hsl_mc64 support optional
One point that still needs addressing is the hardcoded path to the hsl
installation:
target_include_directories(
asterios INTERFACE "$<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}/include>"
#"$<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}/hsl/hsl_mc64-2.3.1/include>"
)
hsl_mc64
does not provide CMake
targets either.