[CMake] Use append for CMAKE_REQUIRED_* variables

[phosek]

Summary

This ensures that required includes and libraries such as -lm that
were added earlier aren't overwritten.

[smeenai]

LGTM

llvm/cmake/config-ix.cmake

if(LLVM_ENABLE_ZLIB STREQUAL FORCE_ON)
     find_package(ZLIB REQUIRED)
   elseif(NOT LLVM_USE_SANITIZER MATCHES "Memory.*")
     find_package(ZLIB)
   endif()
   if(ZLIB_FOUND)
     # Check if zlib we found is usable; for example, we may have found a 32-bit
     # library on a 64-bit system which would result in a link-time failure.
     cmake_push_check_state()
-    set(CMAKE_REQUIRED_INCLUDES ${ZLIB_INCLUDE_DIRS})
-    set(CMAKE_REQUIRED_LIBRARIES ${ZLIB_LIBRARY})
+    list(APPEND CMAKE_REQUIRED_INCLUDES ${ZLIB_INCLUDE_DIRS})
+    list(APPEND CMAKE_REQUIRED_LIBRARIES ${ZLIB_LIBRARY})
     check_symbol_exists(compress2 zlib.h HAVE_ZLIB)
     cmake_pop_check_state()
     if(LLVM_ENABLE_ZLIB STREQUAL FORCE_ON AND NOT HAVE_ZLIB)
       message(FATAL_ERROR "Failed to configure zlib")
     endif()
   endif()
   set(LLVM_ENABLE_ZLIB "${HAVE_ZLIB}")
 endif()
 
 if(LLVM_ENABLE_LIBXML2)
   if(LLVM_ENABLE_LIBXML2 STREQUAL FORCE_ON)
     find_package(LibXml2 REQUIRED)
   elseif(NOT LLVM_USE_SANITIZER MATCHES "Memory.*")
     find_package(LibXml2)
   endif()
   if(LibXml2_FOUND)
     # Check if libxml2 we found is usable; for example, we may have found a 32-bit
     # library on a 64-bit system which would result in a link-time failure.
     cmake_push_check_state()
-    set(CMAKE_REQUIRED_INCLUDES ${LIBXML2_INCLUDE_DIRS})
-    set(CMAKE_REQUIRED_LIBRARIES ${LIBXML2_LIBRARIES})
+    list(APPEND CMAKE_REQUIRED_INCLUDES ${LIBXML2_INCLUDE_DIRS})
+    list(APPEND CMAKE_REQUIRED_LIBRARIES ${LIBXML2_LIBRARIES})
     check_symbol_exists(xmlReadMemory libxml/xmlreader.h HAVE_LIBXML2)
     cmake_pop_check_state()
     if(LLVM_ENABLE_LIBXML2 STREQUAL FORCE_ON AND NOT HAVE_LIBXML2)
       message(FATAL_ERROR "Failed to configure libxml2")
     endif()
   endif()
   set(LLVM_ENABLE_LIBXML2 "${HAVE_LIBXML2}")
 endif()