minor: __GNUC_PREREQ checks __GNUC__ defined

minor: __GNUC_PREREQ checks __GNUC__ defined
(so remove redundant check for defined(__GNUC__))
minor: doc plasma_spin_lock_acquire_spinloop() always returns true
pedantic: identify Solaris with __sun && __SVR4

pedantic: identify Solaris with (defined(__sun) && defined(__SVR4))
http://nadeausoftware.com/articles/2012/01/c_c_tip_how_use_compiler_predefined_macros_detect_operating_system
Solaris NSS compile fixes

**UNTESTED**  Assume unstable!
fix typo in call to xlC intrinsic __stwcx()
clang workarounds to emit C99 extern inline funcs
minor: doc required RPM or dpkg for Python.h
clean-contrib and realclean targets
plasma portability fixes for Solaris, AIX, Win64
consolidate plasma_atomic_fetch_<op>_*_implreturn

consolidate plasma_atomic_fetch_<op>_*_implreturn() macros
rearrange code for add,sub,or,and,xor atomics

rearrange code for add,sub,or,and,xor atomics
- group the specialized implementations
- group the public macro definitions and supporting inline functions
mods for initial port to MacOSX (Darwin)

mcdb 'make test' passes
However, nss_mcdb has not yet been ported to use MacOSX Open Directory
(man opendirectoryd)
plasma_feature.h - OS and architecture features

preprocessor endian detection, largefile support, and more
plasma_stdtypes.h - standard types

portable standard types from sys/types.h, stdint.h, and stdbool.h
simplify code using plasma_{feature,stdtypes}.h