Work around PATH_MAX not defined

Apparently our super-old CentOS 5 containers have weird headers and don't have
PATH_MAX defined when compiling with Clang (but do with gcc?). Who really knows
at this point what's going on, so let's just avoid this entirely as we don't use
dsymutil anyway.

Author: alexcrichton

tools/dsymutil/DwarfLinker.cpp

@@ -1759,10 +1759,12 @@ PointerIntPair<DeclContext *, 1> DeclContextTree::getChildDeclContext(
               (void)gotFileName;
               assert(gotFileName && "Must get file name from line table");
 #ifdef HAVE_REALPATH
+#ifdef PATH_MAX
               char RealPath[PATH_MAX + 1];
               RealPath[PATH_MAX] = 0;
               if (::realpath(File.c_str(), RealPath))
                 File = RealPath;
+#endif
 #endif
               FileRef = StringPool.internString(File);
               U.setResolvedPath(FileNum, FileRef);