Enable osx build on travis

.travis.yml

 dist: xenial
+os:
+  - linux
+compiler:
+  - clang
+  - gcc
 env:
   matrix:
     - CI_BUILD=cmake
     - CI_BUILD=autotools
+matrix:
+  include:
+    - os: osx
+      compiler: clang
+      osx_image: xcode10.2
+      env: CI_BUILD=autotools
 language: cpp
-compiler:
-  - clang
-  - gcc
 sudo: required
 addons:
   apt:
@@ -30,6 +38,13 @@ addons:
     - libc-ares-dev
     - cmake
     - cmake-data
+  homebrew:
+    packages:
+      - libev
+      - libevent
+      - c-ares
+      - cunit
+      - libressl
 before_install:
   - $CC --version
   - if [ "$CXX" = "g++" ]; then export CXX="g++-8" CC="gcc-8"; fi
@@ -37,13 +52,15 @@ before_install:
   - go version
   - cmake --version
 before_script:
+  - if [ "$TRAVIS_OS_NAME" = "linux" ]; then CPPFLAGS=-fsanitize=address LDFLAGS="-fsanitize=address -fuse-ld=gold"; fi
+  - if [ "$TRAVIS_OS_NAME" = "osx" ]; then PKG_CONFIG_PATH=/usr/local/opt/libressl/lib/pkgconfig:/usr/local/opt/libxml2/lib/pkgconfig; fi
   # Now build nghttp2
   - if [ "$CI_BUILD" = "autotools" ]; then autoreconf -i; fi
   - git submodule update --init
-  - if [ "$CI_BUILD" = "autotools" ]; then ./configure --with-mruby; fi
+  - if [ "$CI_BUILD" = "autotools" ]; then ./configure --with-mruby PKG_CONFIG_PATH=$PKG_CONFIG_PATH; fi
   - if [ "$CI_BUILD" = "cmake" ]; then cmake -DENABLE_WERROR=1 -DWITH_MRUBY=1 -DWITH_NEVERBLEED=1; fi
 script:
-  - if [ "$CI_BUILD" = "autotools" ]; then make distcheck DISTCHECK_CONFIGURE_FLAGS="--with-mruby --with-neverbleed --enable-werror CPPFLAGS=-fsanitize=address LDFLAGS=\"-fsanitize=address -fuse-ld=gold\""; fi
+  - if [ "$CI_BUILD" = "autotools" ]; then make distcheck DISTCHECK_CONFIGURE_FLAGS="--with-mruby --with-neverbleed --enable-werror CPPFLAGS=$CPPFLAGS LDFLAGS=\"$LDFLAGS\" PKG_CONFIG_PATH=$PKG_CONFIG_PATH"; fi
   - if [ "$CI_BUILD" = "cmake" ]; then make check; fi
   # As of April, 23, 2016, golang http2 build fails, probably because
   # the default go version is too old.