[libc] [llvm] [libc][cmake][linux] require new LLVM_LIBC_USE_HOST_KERNEL_HEADERS or LIBC_KERNEL_HEADERS (PR #123820)

[Nick Desaulniers via llvm-commits]

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@@ -46,7 +46,11 @@ set(LIBC_BUILD_DIR ${CMAKE_CURRENT_BINARY_DIR})
 
 set(LIBC_ENABLE_USE_BY_CLANG OFF CACHE BOOL "Whether or not to place libc in a build directory findable by a just built clang")
 
-set(LIBC_KERNEL_HEADERS "/usr/include" CACHE STRING "Path to Linux kernel headers")
+# LLVM_LIBC_USE_HOST_KERNEL_HEADERS is brittle, and frowned upon (prefer
+# setting LIBC_KERNEL_HEADERS), but is simpler from a hello world linux host
+# build.
+set(LLVM_LIBC_USE_HOST_KERNEL_HEADERS OFF CACHE BOOL "Add /usr/include to the header seach path")
+set(LIBC_KERNEL_HEADERS "" CACHE STRING "Path to Linux kernel headers")
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nickdesaulniers wrote:

Ok, so it sounds like cmake has distinct "namespace-esque" differentiation between "normal variables" and "cache variables."

For cmake variables that we encourage folks to use to configure llvm-libc, I _think_ we may want `option`. That seems very common in `libcxx/CMakeLists.txt`.

https://github.com/llvm/llvm-project/pull/123820