Add ifndef guards for _LARGEFILE_SOURCE and _LARGEFILE64_SOURCE

This ensures the symbols aren't redefined, which would result in a compiler
error.

I was getting redefined symbols for _LARGEFILE64_SOURCE when building for
32-bit x86 Linux on an older CentOS release in a CI environment. With this
change, I'm able to compile the single file library in this environment.

Closes facebook#2443.

lib/dictBuilder/zdict.c

@@ -23,9 +23,13 @@
 /* Unix Large Files support (>4GB) */
 #define _FILE_OFFSET_BITS 64
 #if (defined(__sun__) && (!defined(__LP64__)))   /* Sun Solaris 32-bits requires specific definitions */
+#  ifndef _LARGEFILE_SOURCE
 #  define _LARGEFILE_SOURCE
+#  endif
 #elif ! defined(__LP64__)                        /* No point defining Large file for 64 bit */
+#  ifndef _LARGEFILE64_SOURCE
 #  define _LARGEFILE64_SOURCE
+#  endif
 #endif