Fix build issues related to Solaris and older GCC

Fixed two build issues:
 - JsonCPP currently doesn’t compile for Solaris due to platform
differences with ‘isfinite’ function.  Fixed by adding proper include
and define for Solaris.
 - JsonCPP currently doesn’t compile for GCC version 4.1.2 and earlier
due to use of ‘-Werror=*’ compile flag, which was introduced in a later
version.  Fixed by adding version check to only add this flag on
supported versions of GCC.