Merge branch 'build-cmake-static-lib' of https://github.com/guilhermeferreira/paho.mqtt.c into fixes

PR #154

Link with OpenSSL crypto library

Signed-off-by: Otavio Rodolfo Piske <opiske@redhat.com>

Do use the OpenSSL crypto library otherwise linking of SSL versions of the libraries fail due to missing symbols
Signed-off-by: Otavio Rodolfo Piske <opiske@redhat.com>

Proposed fix for OpenSSL compilation issue on OS X

Signed-off-by: Guilherme Maciel Ferreira <guilherme.maciel.ferreira@gmail.com>

Use the CMake feature called Object Library. It decreases compilation
by 30%-50%. It avoids compiling the sources more than once.

But one cannot use this feature if there is conditional compilation
flags. For example, it's required to compile the SSL code again
because of the OPENSSL ifdefs.
Signed-off-by: Guilherme Maciel Ferreira <guilherme.maciel.ferreira@gmail.com>

CMake fixes on Microsoft Windows

add missing OPENSSL define to CMakeLists.txt

Signed-off-by: Otavio Rodolfo Piske <opiske@redhat.com>

Signed-off-by: Otavio Rodolfo Piske <opiske@redhat.com>

Switch to cmake build on Travis CI so that it can, more easily, bypass lack of OpenSSL in newer versions of OS X
Signed-off-by: Otavio Rodolfo Piske <opiske@redhat.com>

Apple deprecated OpenSSL and stopped providing its headers on newer versions of OS X. These
changes adjust CMake build files so that they look for Brew's OpenSSL instead (or a custom
one if provided by the user).
For other systems, it uses CMake's own search to find headers/libraries.
Signed-off-by: Otavio Rodolfo Piske <opiske@redhat.com>

Signed-off-by: Otavio Rodolfo Piske <opiske@redhat.com>

Signed-off-by: Ryan Pendleton <me@ryanp.me>

Signed-off-by: Ryan Pendleton <me@ryanp.me>

The pthread library have being added to Windows build. Which
causes linking problems on Visual Studio.
Signed-off-by: Guilherme Maciel Ferreira <guilherme.maciel.ferreira@gmail.com>

Signed-off-by: Guilherme Maciel Ferreira <guilherme.maciel.ferreira@gmail.com>

WIN32 is true when the target system is Microsoft Windows, 32 and
64 bits.

UNIX is true when the target system is Unix or Unix-like (i.e. Apple,
BSD and CYGWIN).
Signed-off-by: Guilherme Maciel Ferreira <guilherme.maciel.ferreira@gmail.com>

Libraries need LIBRARY destination (for shared libraries), and
ARCHIVE for static libraries. Whereas executables need only
RUNTIME destination.
Signed-off-by: Guilherme Maciel Ferreira <guilherme.maciel.ferreira@gmail.com>