Split jobs to two stages in Travis

1. Linter + Doxygen + non-debug Ubuntu/gcc-5 test - linting, documentation
and build on standard OS with standard compilator with non-debug flags.
This will fail fast in case of trivial error and do not waste Travis
resources.
2. Test different OS/CXX/Flags - rest of combinations we support. This
stage will start only if first one will pass.

Author: Vlastimil Zeman

.travis.yml

@@ -1,10 +1,47 @@
 language: cpp
 
-matrix:
+jobs:
   include:
 
+    - &linter-stage
+      stage: Linter + Doxygen + non-debug Ubuntu/gcc-5 test
+      env: NAME="CPP-LINT"
+      install:
+      script: scripts/travis_lint.sh
+      before_cache:
+
+    - stage: Linter + Doxygen + non-debug Ubuntu/gcc-5 test
+      env: NAME="DOXYGEN-CHECK"
+      addons:
+        apt:
+          packages:
+            - doxygen
+      install:
+      script: scripts/travis_doxygen.sh
+      before_cache:
+
+    # Ubuntu Linux with glibc using g++-5
+    - stage: Linter + Doxygen + non-debug Ubuntu/gcc-5 test
+      os: linux
+      sudo: false
+      compiler: gcc
+      cache: ccache
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - libwww-perl
+            - g++-5
+            - libubsan0
+      before_install:
+        - mkdir bin ; ln -s /usr/bin/gcc-5 bin/gcc
+      # env: COMPILER=g++-5 SAN_FLAGS="-fsanitize=undefined -fno-sanitize-recover -fno-omit-frame-pointer"
+      env: COMPILER="g++-5"
+
     # Alpine Linux with musl-libc using g++
-    - os: linux
+    - stage: Test different OS/CXX/Flags
+      os: linux
       sudo: required
       compiler: gcc
       cache: ccache
@@ -17,7 +54,8 @@ matrix:
         - COMPILER="ccache g++"
 
     # OS X using g++
-    - os: osx
+    - stage: Test different OS/CXX/Flags
+      os: osx
       sudo: false
       compiler: gcc
       cache: ccache
@@ -30,7 +68,8 @@ matrix:
       env: COMPILER=g++
 
     # OS X using clang++
-    - os: osx
+    - stage: Test different OS/CXX/Flags
+      os: osx
       sudo: false
       compiler: clang
       cache: ccache
@@ -42,26 +81,9 @@ matrix:
         - COMPILER="ccache clang++ -Qunused-arguments -fcolor-diagnostics"
         - CCACHE_CPP2=yes
 
-    # Ubuntu Linux with glibc using g++-5
-    - os: linux
-      sudo: false
-      compiler: gcc
-      cache: ccache
-      addons:
-        apt:
-          sources:
-            - ubuntu-toolchain-r-test
-          packages:
-            - libwww-perl
-            - g++-5
-            - libubsan0
-      before_install:
-        - mkdir bin ; ln -s /usr/bin/gcc-5 bin/gcc
-      # env: COMPILER=g++-5 SAN_FLAGS="-fsanitize=undefined -fno-sanitize-recover -fno-omit-frame-pointer"
-      env: COMPILER="g++-5"
-
     # Ubuntu Linux with glibc using g++-5, debug mode
-    - os: linux
+    - stage: Test different OS/CXX/Flags
+      os: linux
       sudo: false
       compiler: gcc
       cache: ccache
@@ -82,7 +104,8 @@ matrix:
       script: echo "Not running any tests for a debug build."
 
     # Ubuntu Linux with glibc using clang++-3.7
-    - os: linux
+    - stage: Test different OS/CXX/Flags
+      os: linux
       sudo: false
       compiler: clang
       cache: ccache
@@ -105,7 +128,8 @@ matrix:
         - CCACHE_CPP2=yes
 
     # Ubuntu Linux with glibc using clang++-3.7, debug mode
-    - os: linux
+    - stage: Test different OS/CXX/Flags
+      os: linux
       sudo: false
       compiler: clang
       cache: ccache
@@ -129,25 +153,8 @@ matrix:
         - EXTRA_CXXFLAGS="-DDEBUG"
       script: echo "Not running any tests for a debug build."
 
-    - env: NAME="CPP-LINT"
-      install:
-      script: scripts/travis_lint.sh
-      before_cache:
-
-    - env: NAME="DOXYGEN-CHECK"
-      addons:
-        apt:
-          packages:
-            - doxygen
-      install:
-      script: scripts/travis_doxygen.sh
-      before_cache:
-
   allow_failures:
-    - env: NAME="CPP-LINT"
-      install:
-      script: scripts/travis_lint.sh
-      before_cache:
+    - <<: *linter-stage
 
 install:
   - COMMAND="make -C src minisat2-download" &&