Fix a cmake failure on Windows when LLVM_ENABLE_TERMINFO is set

[lanza]

Summary

The check that uniques these to variables is done under the condition
NOT PURE_WINDOWS and thus LLVM_ENABLE_TERMINFO can be 1 while
TERMINFO_LIB can be undefined. This is just to fix a build failure.

[phosek]

Wasn't this already addressed by https://github.com/llvm/llvm-project/commit/34fe9613dda3c7d8665b609136a8c12deb122382?

[lanza]

Looks like it, I did this around the same time the patch landed. Thanks!

llvm/lib/Support/CMakeLists.txt

if(LLVM_ENABLE_ZLIB)
   endif()
   if(NOT zlib_library)
     get_property(zlib_library TARGET ZLIB::ZLIB PROPERTY LOCATION)
   endif()
   get_system_libname(${zlib_library} zlib_library)
   set(llvm_system_libs ${llvm_system_libs} "${zlib_library}")
 endif()
 
-if(LLVM_ENABLE_TERMINFO)
+if(LLVM_ENABLE_TERMINFO AND TERMINFO_LIB)
   get_system_libname(${TERMINFO_LIB} terminfo_library)
   set(llvm_system_libs ${llvm_system_libs} "${terminfo_library}")
 endif()
 
 set_property(TARGET LLVMSupport PROPERTY LLVM_SYSTEM_LIBS "${llvm_system_libs}")
 
 if(LLVM_WITH_Z3)
   target_include_directories(LLVMSupport SYSTEM
     PRIVATE
     ${Z3_INCLUDE_DIR}
     )
 endif()