[CMake] Toolchain file for TI in CMake 3.10: how do I override compiler options and specify tools?

[Jason Heeris]

I am trying to use CMake (3.10) to build an ANSI C project that may be
compiled on PC with eg. GCC, but also needs to compile with Texas
Instruments' compilers for their microprocessors. So I have about a million
questions.

According to[1] it seems like the way to do this is via a toolchain file.
So I'm trying to write a toolchain file to use TI's compiler/linker/etc,
which do not (always) take the same arguments as eg. GCC or Clang.

Is there a complete list of tools I can override in a toolchain file?
Specifically, I want to set the C compiler, C++ compiler, assembler and
linker. I know about CMAKE_C(XX)_COMPILER, but what about the others? Are
they documented anywhere? (I could guess, but I don't think that's wise.)

As I mentioned, TI's tools aren't the same as GCC, so I need to pare back a
lot of options and start from almost-scratch (there are some
commonalities). Options like "-D" and "-I" are fine, which is good because
then eg. target_include_directories() still works. But certain other flags
are just going to cause errors. How do I completely remove all compile
flags from the generated Makefiles and replace them with my own? I can do
this:

set(CMAKE_C_FLAGS ${MINIMAL_FLAGS} CACHE STRING "" FORCE)
set(CMAKE_C_FLAGS_DEBUG ${MINIMAL_FLAGS} CACHE STRING "" FORCE)
set(CMAKE_C_FLAGS_RELEASE ${MINIMAL_FLAGS} CACHE STRING "" FORCE)
set(CMAKE_C_FLAGS_RELWITHDEBINFO ${MINIMAL_FLAGS} CACHE STRING "" FORCE)
set(CMAKE_C_FLAGS_MINSIZEREL ${MINIMAL_FLAGS} CACHE STRING "" FORCE)

But I *still* see flags in the generated makefiles that I didn't put there
such as "--compile_only" and "--c_file=...". How do I get rid of these and
specify what's correct for my toolchain? (Also, why do I need the CACHE
STRING "" FORCE options? I pulled that out of various Stackoverflow posts
but I have no idea why it's necessary. Is that documented? What about the
configurations... where are they listed? Do I have them all?)

(I keep asking "is this documented anywhere" because I like to provide
references in code for future maintainers. I'm not trying to be unkind.
Maybe once I know enough I can volunteer to write any missing docs myself.)

How do I add default flags that involve the source file name eg. for a file
"main.c" I want to have a C compiler flag "--some_option='main.x'"?

Finally, this really seems like a lot of work despite the fact that
toolchain files are (I thought) meant to be the way to define a different
toolchain. It really feels like I'm swimming against the tide here. Is
there another, more-CMake-ish way to accomplish what I'm trying to do?

[1]
https://gitlab.kitware.com/cmake/community/wikis/doc/cmake/CrossCompiling

Thanks for any help.

- Jason
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