Add macOS and Xcode jobs to the Travis CI matrix.

Change-Id: Ib272919e3fc40e750db2930f9942bfbf6b59dc80
Reviewed-on: https://code-review.googlesource.com/c/re2/+/52672


Reviewed-by: Paul Wankadia <junyer@google.com>

.travis.yml

 language: cpp
-sudo: false
-dist: xenial
-script:
-  - make
-  - make test
-matrix:
+jobs:
   include:
 
     - os: linux
+      dist: xenial
       addons:
         apt:
           sources:
@@ -17,6 +13,7 @@ matrix:
       env:
          - MATRIX_EVAL="CC=gcc-4.8 CXX=g++-4.8"
     - os: linux
+      dist: xenial
       addons:
         apt:
           sources:
@@ -26,6 +23,7 @@ matrix:
       env:
          - MATRIX_EVAL="CC=gcc-4.9 CXX=g++-4.9"
     - os: linux
+      dist: xenial
       addons:
         apt:
           sources:
@@ -35,6 +33,7 @@ matrix:
       env:
          - MATRIX_EVAL="CC=gcc-5 CXX=g++-5"
     - os: linux
+      dist: xenial
       addons:
         apt:
           sources:
@@ -44,6 +43,7 @@ matrix:
       env:
         - MATRIX_EVAL="CC=gcc-6 CXX=g++-6"
     - os: linux
+      dist: xenial
       addons:
         apt:
           sources:
@@ -53,6 +53,7 @@ matrix:
       env:
         - MATRIX_EVAL="CC=gcc-7 CXX=g++-7"
     - os: linux
+      dist: xenial
       addons:
         apt:
           sources:
@@ -62,6 +63,7 @@ matrix:
       env:
         - MATRIX_EVAL="CC=gcc-8 CXX=g++-8"
     - os: linux
+      dist: xenial
       addons:
         apt:
           sources:
@@ -72,6 +74,7 @@ matrix:
         - MATRIX_EVAL="CC=gcc-9 CXX=g++-9"
 
     - os: linux
+      dist: xenial
       addons:
         apt:
           sources:
@@ -83,6 +86,7 @@ matrix:
       env:
         - MATRIX_EVAL="CC=clang-3.8 CXX=clang++-3.8"
     - os: linux
+      dist: xenial
       addons:
         apt:
           sources:
@@ -94,6 +98,7 @@ matrix:
       env:
         - MATRIX_EVAL="CC=clang-3.9 CXX=clang++-3.9"
     - os: linux
+      dist: xenial
       addons:
         apt:
           sources:
@@ -105,6 +110,7 @@ matrix:
       env:
         - MATRIX_EVAL="CC=clang-4.0 CXX=clang++-4.0"
     - os: linux
+      dist: xenial
       addons:
         apt:
           sources:
@@ -116,6 +122,7 @@ matrix:
       env:
         - MATRIX_EVAL="CC=clang-5.0 CXX=clang++-5.0"
     - os: linux
+      dist: xenial
       addons:
         apt:
           sources:
@@ -127,6 +134,7 @@ matrix:
       env:
         - MATRIX_EVAL="CC=clang-6.0 CXX=clang++-6.0"
     - os: linux
+      dist: xenial
       addons:
         apt:
           sources:
@@ -138,6 +146,7 @@ matrix:
       env:
         - MATRIX_EVAL="CC=clang-7 CXX=clang++-7"
     - os: linux
+      dist: xenial
       addons:
         apt:
           sources:
@@ -149,6 +158,7 @@ matrix:
       env:
         - MATRIX_EVAL="CC=clang-8 CXX=clang++-8"
     - os: linux
+      dist: xenial
       addons:
         apt:
           sources:
@@ -160,5 +170,29 @@ matrix:
       env:
         - MATRIX_EVAL="CC=clang-9 CXX=clang++-9"
 
-before_install:
+    - os: osx
+      osx_image: xcode7.3
+      env:
+        - MATRIX_EVAL="CC=clang CXX=clang++"
+    - os: osx
+      osx_image: xcode8.3
+      env:
+        - MATRIX_EVAL="CC=clang CXX=clang++"
+    - os: osx
+      osx_image: xcode9.4
+      env:
+        - MATRIX_EVAL="CC=clang CXX=clang++"
+    - os: osx
+      osx_image: xcode10.3
+      env:
+        - MATRIX_EVAL="CC=clang CXX=clang++"
+    - os: osx
+      osx_image: xcode11.3
+      env:
+        - MATRIX_EVAL="CC=clang CXX=clang++"
+
+before_script:
   - eval "${MATRIX_EVAL}"
+script:
+  - make
+  - make test