[libc] Do not add unittests LLVM_INCLUDE_TESTS is OFF.

[sivachandra]

Hopefully this should fix the problem you reported here: https://reviews.llvm.org/D67867

[nathanchance]

Yes, it does! I can approve this if you need it but I am not an expect in cmake so I cannot comment on how proper this is.

libc/cmake/modules/LLVMLibCRules.cmake

function(add_entrypoint_library target_name)
   add_custom_target(
     ${target_name}
     ALL
     DEPENDS ${library_file}
   )
 endfunction(add_entrypoint_library)
 
 function(add_libc_unittest target_name)
+  if(NOT LLVM_INCLUDE_TESTS)
+    return()
+  endif()
+
   cmake_parse_arguments(
     "LIBC_UNITTEST"
     "" # No optional arguments
     "SUITE" # Single value arguments
     "SRCS;HDRS;DEPENDS" # Multi-value arguments
     ${ARGN}
   )
   if(NOT LIBC_UNITTEST_SRCS)