Fix compile on dist-x86_64-linux builder

Apparently glibc is so old it doesn't have the _POSIX_ARG_MAX constant. This
shouldn't affect anything we use anyway though.

https://travis-ci.org/rust-lang/rust/jobs/399333071

Author: alexcrichton

llvm/lib/Support/Unix/Program.inc

@@ -462,7 +462,7 @@ bool llvm::sys::commandLineFitsWithinSystemLimits(StringRef Program,
   static long ArgMax = sysconf(_SC_ARG_MAX);
   // POSIX requires that _POSIX_ARG_MAX is 4096, which is the lowest possible
   // value for ARG_MAX on a POSIX compliant system.
-  static long ArgMin = _POSIX_ARG_MAX;
+  static long ArgMin = 4096;
 
   // This the same baseline used by xargs.
   long EffectiveArgMax = 128 * 1024;