intptr_t is wrong size on x86_64

[pbcornell]

• intptr_t is an integer that is at least as big as the largest pointer. So, the %d is technically fine on i386 GCC (sizeof(intptr_t) == sizeof(int)), but should be %lld on x86_64 GCC (sizeof(intptr_t) == sizeof(int)). Really, we probably should be using the PRIdPTR macro from inttypes.h to be portable, so this is definitely a bug.

      intptr_t val = ...;
      printf("VALUE: %" PRIdPTR  "\n", val);

• The gmtime_r call is a good call out--the CMake C_STANDARD option with 99 uses -std=gnu99.
• We'll have to evaluate the downgrade of CMake and passing the CFLAGS in, I'm not sure this is the right thing to do.
• Consider requiring use of CMake >= 3.5 (note that 3.5 is not available on Ubuntu 14.04).