produce incorrect results for ROUND() 

Added a workaround and a configure check to test whether 
isinf() is affected by the GCC bug #39228. 
       
Since no code in MySQL server is currently affected by that 
bug, the patch is actually a safeguard for possible future 
code modifications. No test cases or changelog entries are 
needed. 


configure.in:
  Added a configure check to test whether isinf() is safe to use
  in C code.
include/my_global.h:
  Added a workaround for GCC bug #39228.