Solution: Modified CMakelist.txt to build correctly for Windows
- corrected CMake required version to make sure it builds in
  Ubuntu 14.04 LTS with no warnings.

Solution: Modified CMakelist.txt to build correctly for Windows

Problem: Windows Build not using precompiled headers for all targets

Problem: Added "make clean" to Windows build scripts

Solution: Enabled precompiled header settings in all targets
Before build time: ~6min 49 secs
Stop  Time: Fri 03/04/2016  8:29:50.13
Start Time: Fri 03/04/2016  8:23:00.28

After build time: ~4min 19 secs
Stop  Time: Fri 03/04/2016 12:12:10.24
Start Time: Fri 03/04/2016 12:07:51.78

Solution: Added ability to do equivalent of a "make clean" by executing:

O:\git\libzmq\builds\msvc\build>buildall.bat clean
Start Time: Fri 03/04/2016 14:33:56.50
Cleaning without libsodium
Cleaning ..\vs2013\libzmq.sln... ()
Platform=x86
Configuration=DynDebug
Configuration=DynRelease
Configuration=LtcgDebug
Configuration=LtcgRelease
Configuration=StaticDebug
Configuration=StaticRelease
Platform=x64
Configuration=DynDebug
Configuration=DynRelease
Configuration=LtcgDebug
Configuration=LtcgRelease
Configuration=StaticDebug
Configuration=StaticRelease
Cleaning complete: ..\vs2013\libzmq.sln

Stop  Time: Fri 03/04/2016 14:34:01.84
Start Time: Fri 03/04/2016 14:33:56.50

A clean takes ~5 secs to complete.

Problem: Visual Studio Toolset was incorrectly set in VCXPROJ files

Problem: Fix C++ style comment usage in C source

Problem: Fix unportable `==` operator for `test`

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.