The current check to define JSON_USE_INT64_DOUBLE_CONVERSION
works for GCC but not clang.
Clang does define __GNUC__ but with a value 4 which misses
the check for >= 6.
This avoids the -Wimplicit-int-float-conversion warning
when jsoncpp is built with a recent version of clang.
Signed-off-by: Manoj Gupta <manojgupta@google.com>
3013ed48b3