Upcoming breaking BLT changes release

[white238]

BLT and CMake has grown a lot since BLT's creation. It was always the intention to avoid breaking changes but it is time to mature along with CMake. The intention of this is to discuss and socialize changes.

• Remove BLT_DEFINES
• Remove BLT language flags: BLT_C_FLAGS, BLT_CXX_FLAGS, BLT_CUDA_FLAGS, and BLT_FORTRAN_FLAGS
• Remove BLT_EXE_LINKER_FLAGS
• Use GNUInstallDirs and remove creating generic paths here
• Remove CUDA_LINK_WITH_NVCC and CUDA_SEPARABLE_COMPILATION (note: these are non-prefixed BLT variables)
• Remove blt_register_library()
• Remove BLT_EXPORT_THIRDPARTY and blt_export_tpl_targets()
• Guard all BLT options with BLT_: ENABLE_CUDA, ENABLE_TESTING, etc.
• Remove enabling auto-detected tools: YAPF, clang-format, etc.
• Remove TARGET from blt_convert_to_system_includes

Things that need debating:

• Remove alias targets and just have blt_ prefixed targets, ie, cuda and blt::cuda will just be blt_cuda
• Remove builtin TPLs (GoogleTest, GoogleBenchMark, GoogleMock, FRUIT)
• Remove BLT_CXX_STD

[rhornung67]

What about BLT_CXX_STD?

[white238]

I'm torn on that one, it makes sure three variables are consistent: CMAKE_CXX_STANDARD, CMAKE_CUDA_STANDARD, CMAKE_HIP_STANDARD. Maybe renamed? Though maybe its better to just set all three independently.

[white238]

Though maybe in BLT we just set the CUDA and HIP variables to the CXX one if the user doesn't....

[adayton1]

I think we should also prefix all the options (e.g. BLT_ENABLE_CUDA instead of ENABLE_CUDA). I've seen a lot of projects use the no-prefix style with their own variables and it gets confusing fast which variables affect which project, and there is a high potential for name conflicts.