OpenBSD defines Elf64_Dyn::d_tag to be of an unsigned type, which differs from
what most other platforms do and what is the case for 32 bit. To have the tag
as unsigned makes sense, but this causes a compilation warning:

/.../CMake/Source/cmELF.cxx: In member function 'const cmELF::StringEntry* cmELFInternalImpl<Types>::GetDynamicSectionString(int) [with Types = cmELFTypes64]':
/.../CMake/Source/cmELF.cxx:945:   instantiated from here
/.../CMake/Source/cmELF.cxx:668: warning: comparison between signed and unsigned integer expressions

Add an explicit typedef to cast the value to for 32 and 64 bit. That type is
unsigned and has the proper length for both platforms so no information is
lost. Explicitely cast both arguments before comparing them to avoid the
warning in all situations.