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

2, Verifies the EnumValueDscriptor for DynamicMessage.Builder#setField

Increase the allocated_size_ should be after allocation. Otherwise there might have segmentation fault if allocation throws an exception.

"unary minus operator applied to unsigned type, result still unsigned"