BUG: Using static const variable causes ld error when using dynamic libraries

Static const variables yeld STB_GNU_UNIQUE symbols in shared libraries, and prevent these libraries to be properly unloaded. As a result, a program using dynamic libraries loading including calls to itk::MattesMutualInformationImageToImageMetric will return the following error whenever it ends :

Inconsistency detected by ld.so: dl-close.c: 731: _dl_close: Assertion `map->l_init_called' failed!

This patch only replaces the static const variables by const variables.

The pre-commit hook made me do the style changes. I applied the requested changes in this new version.

Change-Id: I5862ff5a965599dbf2a4cc1b48b5a4931588653b
Merge topic 'static_const_removal'

d74f4d8 BUG: Using static const variable causes ld error when using dynamic libraries