Upmerge changes done to the generic spec file in 5.0 to 5.1,

this includes a major whitespace (formatting) alignment
and sequence changes to better agree with other spec files.

Further changes:
- All features are controlled by "%define" set from call
  options or builtin.
- "bundled zlib" is on by default.
- "with libgcc" is controlled by runtime detection of gcc.
- Handling of "CFLAGS" and "CXXFLAGS" is more concentrated.
- Several missing man pages were added.