freebsd: malloc_usable_size check malloc_np.h (#7009)

Summary:
Per https://www.unix.com/man-page/freebsd/3/malloc_usable_size/
malloc_usable_size is in malloc_np.h as its a non-standard API.

Without patch it just fails to detect from ./CMakeFiles/CMakeError.log

In file included from /home/dan/build-rocksdb/CMakeFiles/CMakeTmp/CheckSymbolExists.cxx:2:
/usr/include/malloc.h:3:2: error: "<malloc.h> has been replaced by <stdlib.h>"
 ^
/home/dan/build-rocksdb/CMakeFiles/CMakeTmp/CheckSymbolExists.cxx:8:19: error: use of undeclared identifier 'malloc_usable_size'
  return ((int*)(&malloc_usable_size))[argc];
                  ^
2 errors generated.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7009

Reviewed By: riversand963

Differential Revision: D22176093

Pulled By: ajkr

fbshipit-source-id: da980f3d343b6d9b0c70d7827c6df495f3fb1ade

CMakeLists.txt

@@ -511,7 +511,11 @@ if(HAVE_PTHREAD_MUTEX_ADAPTIVE_NP)
 endif()
 
 include(CheckCXXSymbolExists)
-check_cxx_symbol_exists(malloc_usable_size malloc.h HAVE_MALLOC_USABLE_SIZE)
+if(CMAKE_SYSTEM_NAME MATCHES "^FreeBSD")
+  check_cxx_symbol_exists(malloc_usable_size malloc_np.h HAVE_MALLOC_USABLE_SIZE)
+else()
+  check_cxx_symbol_exists(malloc_usable_size malloc.h HAVE_MALLOC_USABLE_SIZE)
+endif()
 if(HAVE_MALLOC_USABLE_SIZE)
   add_definitions(-DROCKSDB_MALLOC_USABLE_SIZE)
 endif()