[libunwind] [cmake] Support overriding LLVM_CMAKE_PATH

[mgorny]

Summary

Make LLVM_CMAKE_PATH a cache variable in order to support user
overrides. This is useful when building multiple multilib variants of
libunwind using CMake files from a single-ABI install of LLVM, e.g. when
LLVM_LIBDIR_SUFFIX == "32" but CMake files are to be found in
/usr/lib64/cmake/llvm. Since libunwind is not using any of the LLVM
build configuration variables (just generic macros), it is fine to use
CMake files from a different ABI.

[mgorny]

Since D26894 was accepted, I will be preparing a patch using that instead.

[mgorny]

@EricWF suggests that it'd be better to wait a while before depending on newer llvm-config, so it'd probably be preferable to do this one instead first.

[mgorny]

And the original variant landed in after all. Sorry for the noise.

CMakeLists.txt

elseif(LLVM_CONFIG_PATH)
 
     list(GET CONFIG_OUTPUT 0 INCLUDE_DIR)
     list(GET CONFIG_OUTPUT 1 LLVM_OBJ_ROOT)
     list(GET CONFIG_OUTPUT 2 MAIN_SRC_DIR)
 
     set(LLVM_INCLUDE_DIR ${INCLUDE_DIR} CACHE PATH "Path to llvm/include")
     set(LLVM_BINARY_DIR ${LLVM_OBJ_ROOT} CACHE PATH "Path to LLVM build tree")
     set(LLVM_MAIN_SRC_DIR ${MAIN_SRC_DIR} CACHE PATH "Path to LLVM source tree")
-    set(LLVM_CMAKE_PATH "${LLVM_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX}/cmake/llvm")
+    set(LLVM_CMAKE_PATH "${LLVM_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX}/cmake/llvm"
+        CACHE PATH "Path to LLVM CMake modules")
     set(LLVM_LIT_PATH "${LLVM_PATH}/utils/lit/lit.py")
   else()
     message(FATAL_ERROR "llvm-config not found and LLVM_MAIN_SRC_DIR not defined. "
                         "Reconfigure with -DLLVM_CONFIG=path/to/llvm-config "
                         "or -DLLVM_PATH=path/to/llvm-source-root.")
   endif()
 
   if (EXISTS ${LLVM_CMAKE_PATH})