Patches from vtk (#358)

* Drop the export attribute on forward declaration

They don't mean anything on forward declarations anyways.

* Fixed -Wunused-parameter warnings in some C++ files

Allowed all the attribute #defines to work in C++ also.

* Fixed -Wunused-variable warnings by removing dead code

* Fixed all -Wshorten-64-to-32 warnings by adjusting casts

There was no truncation in fact, since the unsigned result was upcast to size_t then implicitly downcast to uint32_t.

* Fixed all -Wcomma warnings by spliting statements onto own lines

* Fixed all -Wself-assign warnings with different unused param suppression

* Fixed all -Wextra-semi warnings

* Fixed all -Wswitch-enum warnings

Just added new explicit cases with the existing default case.

* Fixed all -Wmissing-prototypes warnings

Just made functions static. Moved them into an extern "C" block.

* Reformatted source with bin/format_source using clang version 10.0.1

Co-authored-by: Larry Knox <lrknox@hdfgroup.org>

tools/lib/h5diff_array.c

@@ -472,9 +472,7 @@ diff_datum(void *_mem1, void *_mem2, hsize_t elemtno, diff_opt_t *opts, hid_t co
     size_t         size = 0;
     hbool_t        iszero1;
     hbool_t        iszero2;
-    hsize_t        nfound = 0; /* differences found */
-    double         per;
-    hbool_t        both_zero;
+    hsize_t        nfound    = 0; /* differences found */
     diff_err_t     ret_value = opts->err_stat;
 
     H5TOOLS_START_DEBUG("ph:%d elemtno:%d - errstat:%d", opts->print_header, elemtno, opts->err_stat);