[cmake] Only attempt to install MSVC system libraries on Windows

[smeenai]

Summary

Newer versions of CMake (I'm on 3.10, but I believe 3.9 behaves the same
way) attempt to query the system for information about the VS 2017
install. Unfortunately, this query fails on non-Windows systems:

cmake_host_system_information does not recognize <key> VS_15_DIR

CMake isn't going to find these system libraries on non-Windows anyway
(and we were previously silencing the resultant warnings in our
cross-compilation toolchain), so it makes sense to just omit the
attempted installation entirely on non-Windows.

llvm/trunk/CMakeLists.txt

else()
       message(SEND_ERROR "Specified distribution component '${target}' doesn't have an install-stripped target."
                          " Its installation target creation should be changed to use add_llvm_install_targets,"
                          " or you should manually create the 'install-${target}-stripped' target.")
     endif()
   endforeach()
 endif()
 
 # This allows us to deploy the Universal CRT DLLs by passing -DCMAKE_INSTALL_UCRT_LIBRARIES=ON to CMake
-if (MSVC)
+if (MSVC AND CMAKE_HOST_SYSTEM_NAME STREQUAL "Windows")
   include(InstallRequiredSystemLibraries)
 endif()
 

llvm/trunk/cmake/platforms/WinMsvc.cmake

 set(_CMAKE_SHARED_LINKER_FLAGS_INITIAL "${CMAKE_SHARED_LINKER_FLAGS}" CACHE STRING "")
 set(CMAKE_SHARED_LINKER_FLAGS "${_CMAKE_SHARED_LINKER_FLAGS_INITIAL} ${LINK_FLAGS}" CACHE STRING "" FORCE)
 
 # CMake populates these with a bunch of unnecessary libraries, which requires
 # extra case-correcting symlinks and what not. Instead, let projects explicitly
 # control which libraries they require.
 set(CMAKE_C_STANDARD_LIBRARIES "" CACHE STRING "" FORCE)
 set(CMAKE_CXX_STANDARD_LIBRARIES "" CACHE STRING "" FORCE)
-
-# CMake's InstallRequiredSystemLibraries module searches for a Visual Studio
-# installation in order to determine where to copy the required DLLs. This
-# installation won't exist when cross-compiling, of course, so silence the
-# resulting warnings about missing libraries.
-set(CMAKE_INSTALL_SYSTEM_RUNTIME_LIBS_NO_WARNINGS ON)
-