[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:
http://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20160926/171659.html

llvm-svn: 293468

openmp/libomptarget/src/omptarget.h

@@ -198,6 +198,7 @@ void __kmpc_push_target_tripcount(int32_t device_id, uint64_t loop_tripcount);
     fprintf(stderr, __VA_ARGS__);                                              \
   }
 
+#define __STDC_FORMAT_MACROS
 #include <inttypes.h>
 #define DPxMOD "0x%0*" PRIxPTR
 #define DPxPTR(ptr) ((int)(2*sizeof(uintptr_t))), ((uintptr_t) (ptr))