- Since isinf() portability across various platforms and
  compilers is a complicated question, we should not use
  it directly. Instead, the my_isinf() macro should be used,
  which is defined as an alias to the system-defined isinf()
  if it is safe to use, or a workaround implementation otherwise


configure.in:
  Added a check to define HAVE_ISINF only if it can be used
  in C++ code as well.
include/my_global.h:
  Define my_isinf() as an alias to isinf(), if it is available
  in both C and C++ code. Otherwise, define it to a workaround
  implementation.
sql/item_func.cc:
  Replaced isinf() with my_isinf().
strings/strtod.c:
  Replaced isinf() with my_isinf().