Problem: Fix build with Heimdal krb5

Solution:
* Check gssapi/gssapi_generic.h header file, it is not in Heimdal.
* Check libgssapi too, libgssapi_krb5 is not separated in Heimdal.

minor automake fixes plus tweetnacl logic change

Brackets around defaults in configure --help strings for
--with-libsodium and --disable-curve were not displayed.

Solution:  Add m4 quotes.

Move AM_CONDITIONAL for --disable-curve outside of shell
conditional (per sec 20.1 of automake manual) and fix its
second argument to be a test rather than a literal zero.

Problem: can't be sure crypto_box always returns 0

Libsodium has started returning -1 in some cases.

Solution: allow and handle error returns from these calls.

Fixes #1831

include sys/ucred.h for struct ucred

Platforms that have struct ucred, typically declare it in sys/ucred.h

update Makefile.mingw32

Problem: make dist is borken

Solution: add it to .gitignore

Solution: change builds/msvc/Makefile.am to reference vcxproj files
instead of props files.

Solution: change builds/Makefile.am to reference builds/android
instead of builds/qt-android.

Solution: remove references to builds/cmake/Makefile* since they no
longer exist.

Solution: socket_base_t::in_event cannot do anything useful with
return status of process_commands. Asserting is the wrong solution,
as it is entirely valid to be interrupted or for the context to be
terminated, so discard the value.

Solution: use NULL instead to avoid build failure with older
compilers.

Solution: initialize class variable in the same order as they are
defined.

Solution: initialize to -1, since fd is a file descriptor (int).

Solution: initialize class variable in the same order as they are
defined.

Solution: initialize class variable in the same order as they are
defined.

Solution: initialize class variable in the same order as they are
defined.

Solution: initialize class variable in the same order as they are
defined.

Solution: initialize class variable in the same order as they are
defined.

Solution: initialize class variable in the same order as they are
defined.

Solution: initialize class variable in the same order as they are
defined.

Solution: don't store it in zmq::poller_base_t::adjust_load, as the
build with Werror=unused=variable will fail otherwise.

Solution: remove statc initialization to NULL of thread.hpp pthread_t
descriptor. There is no portable way to statically initialize a
pthread_t variable.

Solution: initialise it inside an ifdef too

Solution: fix it

Problem: Several problems found by Coverity Static Analyzer

Solution: The Coverity Static Code Analyzer was used on libzmq code and found
many issues with uninitialized member variables, some redefinition of variables
hidding previous instances of same variable name and a couple of functions
where return values were not checked, even though all other occurrences were
checked (e.g. init_size() return).

Problem: Visual Studio Toolset was incorrectly set in VCXPROJ files

Solution: Corrected Toolset setting where needed and inprove compilation speed
by adding defintion of WIN32_LEAN_AND_MEAN prior to any Windows specific
include files, which skips non-essential definitions during compilation.

Bump C/C++ standard version support to C11/C++11

Solution: remove it