Travis: Move the OSX GCC build to the first stage as well

We can run up to 10 jobs in parallel - make use of that!

Author: tautschnig

.travis.yml

@@ -4,7 +4,7 @@ jobs:
   include:
 
     - &formatting-stage
-      stage: Linter + Doxygen + non-debug Ubuntu/gcc-5 build
+      stage: Linter + Doxygen + non-debug GCC builds
       env: NAME="clang-format"
       addons:
         apt:
@@ -32,20 +32,20 @@ jobs:
       before_cache:
 
     - &linter-stage
-      stage: Linter + Doxygen + non-debug Ubuntu/gcc-5 build
+      stage: Linter + Doxygen + non-debug GCC builds
       env: NAME="CPP-LINT"
       install:
       script: scripts/travis_lint.sh
       before_cache:
 
     - &string-table-check
-      stage: Linter + Doxygen + non-debug Ubuntu/gcc-5 build
+      stage: Linter + Doxygen + non-debug GCC builds
       env: NAME="string-table"
       install:
       script: scripts/string_table_check.sh
       before_cache:
 
-    - stage: Linter + Doxygen + non-debug Ubuntu/gcc-5 build
+    - stage: Linter + Doxygen + non-debug GCC builds
       env: NAME="DOXYGEN-CHECK"
       addons:
         apt:
@@ -69,7 +69,7 @@ jobs:
         - scripts/publish_doc.sh
 
     # Ubuntu Linux with glibc using g++-5
-    - stage: Linter + Doxygen + non-debug Ubuntu/gcc-5 build
+    - stage: Linter + Doxygen + non-debug GCC builds
       os: linux
       sudo: false
       compiler: gcc
@@ -92,7 +92,7 @@ jobs:
       script: echo "Tests are run in the next stage."
 
     # cmake build using g++-5
-    - stage: Linter + Doxygen + non-debug Ubuntu/gcc-5 build
+    - stage: Linter + Doxygen + non-debug GCC builds
       os: linux
       compiler: gcc
       cache: ccache
@@ -113,6 +113,18 @@ jobs:
         - cmake --build build -- -j4
       script: echo "Tests are run in the next stage."
 
+    # OS X using g++
+    - stage: Linter + Doxygen + non-debug GCC builds
+      os: osx
+      sudo: false
+      compiler: gcc
+      cache: ccache
+      before_install:
+        - HOMEBREW_NO_AUTO_UPDATE=1 brew install ccache parallel
+        - export PATH=$PATH:/usr/local/opt/ccache/libexec
+      env: COMPILER="ccache g++"
+      script: echo "Tests are run in the next stage."
+
     # Ubuntu Linux with glibc using g++-5
     - stage: Test different OS/CXX/Flags
       os: linux