[cmake] Only pass CMAKE_SYSROOT if non-empty

[smeenai]

Summary

In my build environment (cmake 3.6.1 and gcc 4.8.5 on CentOS 7), having
an empty CMAKE_SYSROOT in the cache results in --sysroot="" being passed
to all compile commands, and then the compiler errors out because of the
empty sysroot. Only set CMAKE_SYSROOT if non-empty to avoid this.

[beanz]

LGTM

llvm/trunk/cmake/modules/LLVMExternalProjectUtils.cmake

function(llvm_ExternalProject_Add name source_dir)
 
   add_custom_target(${name}-clobber
     DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/${name}-clobber-stamp)
 
   if(ARG_EXCLUDE_FROM_ALL)
     set(exclude EXCLUDE_FROM_ALL 1)
   endif()
 
+  if(CMAKE_SYSROOT)
+    set(sysroot_arg -DCMAKE_SYSROOT=${CMAKE_SYSROOT})
+  endif()
+
   ExternalProject_Add(${name}
     DEPENDS ${ARG_DEPENDS} llvm-config
     ${name}-clobber
     PREFIX ${CMAKE_BINARY_DIR}/projects/${name}
     SOURCE_DIR ${source_dir}
     STAMP_DIR ${STAMP_DIR}
     BINARY_DIR ${BINARY_DIR}
     ${exclude}
     CMAKE_ARGS ${${nameCanon}_CMAKE_ARGS}
                ${compiler_args}
                -DCMAKE_INSTALL_PREFIX=${CMAKE_INSTALL_PREFIX}
-               -DCMAKE_SYSROOT=${CMAKE_SYSROOT}
+               ${sysroot_arg}
                -DLLVM_BINARY_DIR=${PROJECT_BINARY_DIR}
                -DLLVM_CONFIG_PATH=$<TARGET_FILE:llvm-config>
                -DLLVM_ENABLE_WERROR=${LLVM_ENABLE_WERROR}
                -DLLVM_HOST_TRIPLE=${LLVM_HOST_TRIPLE}
                -DPACKAGE_VERSION=${PACKAGE_VERSION}
                -DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE}
                -DCMAKE_MAKE_PROGRAM=${CMAKE_MAKE_PROGRAM}
                -DCMAKE_EXPORT_COMPILE_COMMANDS=1