[libomptarget] Fix Debug build with glibc < 2.18
glibc < 2.18 is C99 compliant and only provides the format macros in C++ if
__STDC_FORMAT_MACROS is defined. This change fixes the debug build for
GCC 4.8, GCC 6.2 and Clang 3.9.1 that were previously broken on my machine.
It shows no regression for libc++ >= 4.0.0 which has a fix since September: