[cmake] Only run llvm-codesign if targetting apple on an apple host

[lanza]

Summary

Other platforms don't have the capability to perform llvm_codesign
step. If LLVM_CODESIGNING_IDENTITY is set then this chunk of code would
attempt to codesign if the target was Apple. But when cross compiling
to Darwin from Linux, for example, this step would fail. So test if the
host is Apple as well.

[lanza]

@JDevlieghere is this what you had in mind?

[JDevlieghere]

Yup, thank you Nathan. This LGTM

[lanza]

rebase

[sgraenitz]

Was this tested? I am not getting APPLE here:

$ uname -s
Darwin

https://cmake.org/cmake/help/v3.4/variable/CMAKE_HOST_SYSTEM_NAME.html says:

On systems that have the uname command, this variable is set to the output of uname -s. Linux, Windows, and Darwin for Mac OS X are the values found on the big three operating systems.

What motivated the decision against CMAKE_HOST_APPLE?
https://cmake.org/cmake/help/v3.4/variable/CMAKE_HOST_APPLE.html

[lanza]

Yup! Sorry about that, this was fixed in git commit 474ca495a914fca0e.

llvm/trunk/cmake/modules/AddLLVM.cmake

if(CMAKE_GENERATOR STREQUAL "Xcode")
     set_target_properties(${name} PROPERTIES
       XCODE_ATTRIBUTE_CODE_SIGN_IDENTITY ${LLVM_CODESIGNING_IDENTITY}
     )
     if(DEFINED ARG_ENTITLEMENTS)
       set_target_properties(${name} PROPERTIES
         XCODE_ATTRIBUTE_CODE_SIGN_ENTITLEMENTS ${ARG_ENTITLEMENTS}
       )
     endif()
-  elseif(APPLE)
+  elseif(APPLE AND CMAKE_HOST_SYSTEM_NAME MATCHES APPLE)
     if(NOT CMAKE_CODESIGN)
       set(CMAKE_CODESIGN xcrun codesign)
     endif()
     if(NOT CMAKE_CODESIGN_ALLOCATE)
       execute_process(
         COMMAND xcrun -f codesign_allocate
         OUTPUT_STRIP_TRAILING_WHITESPACE
         OUTPUT_VARIABLE CMAKE_CODESIGN_ALLOCATE