[TravisCI] Add scripts to build and test Z3 on macOS (OSX) and

add a single configuration to TravisCI to test.

TravisCI is very slow at running macOS jobs so just have one
configuration for now.

.travis.yml

@@ -59,11 +59,16 @@ env:
     # 64-bit GCC 4.8 Debug
     - LINUX_BASE=ubuntu_14.04 C_COMPILER=/usr/bin/gcc-4.8 CXX_COMPILER=/usr/bin/g++-4.8 TARGET_ARCH=x86_64 Z3_BUILD_TYPE=Debug
 
-# TODO: OSX support
-#matrix:
-#  include:
-#    - os: osx
-#      osx_image: xcode 8.2
+# macOS (a.k.a OSX) support
+matrix:
+  include:
+    # For now just test a single configuration. macOS builds on TravisCI are
+    # very slow so we should keep the number of configurations we test on this
+    # OS to a minimum.
+    - os: osx
+      osx_image: xcode8.3
+      # Note: Apple Clang does not support OpenMP
+      env: Z3_BUILD_TYPE=RelWithDebInfo USE_OPENMP=0
 script:
   # Use `travis_wait` when doing LTO builds because this configuration will
   # have long link times during which it will not show any output which