platform_macros.h: #undef GOOGLE_PROTOBUF_PLATFORM_ERROR once it's no longer needed

Fix atomicops build failure on non-Clang

We cannot use Clang's __has_extension macro unless we really are
compiling on Clang, which means we cannot use this expression:

    #if (defined(__clang__) && __has_extension(c_atomic)))
    // ...
    #endif

On GCC, this generates the following errors:

    In file included from ./google/protobuf/stubs/atomicops.h:59:0,
                     from google/protobuf/stubs/atomicops_internals_x86_gcc.cc:36:
    ./google/protobuf/stubs/platform_macros.h:67:41: error: missing binary operator before token "("
       (defined(__clang__) && __has_extension(c_atomic)))
                                             ^
    In file included from google/protobuf/stubs/atomicops_internals_x86_gcc.cc:36:0:
    ./google/protobuf/stubs/atomicops.h:196:40: error: missing binary operator before token "("
      (defined(__clang__) && __has_extension(c_atomic))
                                            ^

Instead, we have to protect the __has_extension expression by only
executing it when __clang__ is defined:

    #if defined(__clang__)
    # if __has_extension(c_atomic)
    // ...
    # endif
    #endif

Expose generic atomicops on Clang

Remove GOOGLE_PROTOBUF_ARCH_PPC

The generic atomicops implementation is only exposed if GCC >= 4.7 is
available, but Clang, where the underlying __atomic built-ins are also
available, typically only claims to be GCC 4.2. This causes build
failures when compiling protobuf or the output of protoc's C++ code
generator on an architecture that needs the generic atomicops
implementation with Clang.

Clang has a "c_atomic" extension which can be tested for which almost
does what we want:

    C11 atomic operations

    Use __has_feature(c_atomic) or __has_extension(c_atomic) to
    determine if support for atomic types using _Atomic is enabled.
    Clang also provides a set of builtins which can be used to implement
    the <stdatomic.h> operations on _Atomic types.

I'm not sure if this guarantees that the GNU atomic builtins (the ones
with the __atomic prefix) are also available, but in practice this
should guarantee that Clang is new enough.

With this change in place, Clang generates several diagnostics when
compiling the generic atomicops implementation. These appear to be bugs
in the generic atomicops implementation and are not Clang-specific.

The macro GOOGLE_PROTOBUF_ARCH_PPC is not used anywhere in the protobuf
source; there is no Power-specific atomics implementation, etc.

Funnily enough, the macro __ppc__ is not actually defined on 32-bit
Power on GCC/Linux, according to the following webpage:

    http://nadeausoftware.com/articles/2012/02/c_c_tip_how_detect_processor_type_using_compiler_predefined_macros#POWER

and verified on a 32-bit Debian sid 'powerpc' chroot:

    (sid_powerpc-dchroot)edmonds@partch:~$ gcc -dM -E - < /dev/null | grep -c __ppc__
    0
    (sid_powerpc-dchroot)edmonds@partch:~$ gcc -dM -E - < /dev/null | grep -c __LP64__
    0

Bump version for maven-bundle-plugin

remove a const qualifier in a method's return type

fix a compile warning

This change fixes the following compiler warning:

warning: type qualifiers ignored on function return type [-Wignored-qualifiers]

Added const qualifier to iterator to enable compiling with VS2008

Rename README.txt to README.md and use markdown formatting.

Add instructions on how to generate the configure script.

in README.txt.

patch Issue 130740043 to trunk for other atomicops patches: [MIPS64] Add support for MIPS64 in protobuf

make dist complains the file name is too long and refuse to put it to tar. Change the file to a shorter name

1, Add comments for include<algorithm> we added for min/max on VS2013. 2, Add #if/#endif to wrap #undef

Cygwin doesn't work well if we try to capture stderr and stdout at the same time. Comment one test tempraly for Cygwin. Need to figure out why and add the test back for Cygwin.

With AM_PROG_AR may report "error: possibly underfined macro: AM_PROG_AR" (mingw, autoreconf-2.68).
Without AM_PROG_AR may report warning "archiver requires 'AM_PROG_AR' in 'configure.ac'" (cygwin, automake-1.12).
They do not affect the tests and install