Travis-CI: Mac OS X build improvements

- Properly define build dependencies via addons/homebrew, but still keep
  secondary Homebrew run until OS X builds fully work.
- Remove references to OS X bugs that are already closed.
- As long as the OS X build does not work, it is enough to attempt to run
  just one of them, no need for many in parallel. It will just waste
  resources and slow down the job from finishing quickly.

.travis.yml

@@ -8,8 +8,6 @@ git:
 
 language: cpp
 
-osx_image: xcode10.1
-
 cache:
   timeout: 500
   apt: true
@@ -66,15 +64,42 @@ addons:
       - psmisc
       - uuid-dev
       - zlib1g-dev
-
-# libsystemd-daemon-dev # https://github.com/travis-ci/apt-package-whitelist/issues/3882
+      # libsystemd-daemon-dev # https://github.com/travis-ci/apt-package-whitelist/issues/3882
+  homebrew:
+    # Temporary workaround, remove when this is solved:
+    # https://travis-ci.community/t/macos-build-fails-because-of-homebrew-bundle-unknown-command/7296/20
+    update: true
+    # OSX builds are not properly maintained on Travis-CI nor buildbot and
+    # thus this list of dependencies is likely to be inaccurate.
+    packages:
+      - bison
+      - boost
+      - ccache
+      - cmake
+      - cracklib
+      - curl
+      - gnutls
+      - groonga
+      - jemalloc
+      - libxml2
+      - lz4
+      - lzo
+      - ncurses
+      - openssl@1.1
+      - ossp-uuid
+      - pcre2
+      - perl
+      - pkg-config
+      - readline
+      - snappy
+      - xz
+      - zlib
+      - zstd
 
 # The jobs matrix consists of all the combinations of 'arch', 'os', 'compiler' and 'env'
 arch:
   - amd64
-os:
-  - linux
-  - osx
+os: linux
 compiler:
   - gcc
   - clang
@@ -90,10 +115,6 @@ env:
 jobs:
   # Define what of the above defined jobs matrix is excluded
   exclude:
-    - os: osx
-      compiler: gcc
-    - os: osx
-      env: CC_VERSION=8 TYPE=RelWithDebInfo MYSQL_TEST_SUITES=main
     - compiler: clang
       env: CC_VERSION=8 TYPE=RelWithDebInfo MYSQL_TEST_SUITES=main
 
@@ -129,6 +150,11 @@ jobs:
       env: DebPackages=bionic
       addons: *deb-addons
       script: *deb-script
+      # Just one OS X build is enough
+    - os: osx
+      osx_image: xcode10.1
+      compiler: clang
+      env: CC_VERSION=8 TYPE=RelWithDebInfo MYSQL_TEST_SUITES=main
       # Some chosen ones with s390x and gcc
     - os: linux
       arch: s390x
@@ -202,7 +228,7 @@ jobs:
       arch: amd64
       compiler: clang
       env: CC_VERSION=8 TYPE=RelWithDebInfo MYSQL_TEST_SUITES=main
-    # Until OSX becomes a bit more stable: MDEV-12435 MDEV-16213
+    # Until OSX becomes a bit more stable
     - os: osx
 
 before_install: