Fix: Define __STDC_LIMIT_MACROS to fix C++ builds

Necessary to include the fixed-width type limits on glibc versions
older than 2.18 when building with a C++ compiler.

Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>