BUILD: Only install packages on Travis if we're actually building

All but one Travis build on the Coverity Scan branch bail before
compilation. We don't need to install packages there then.

.travis.yml

@@ -15,59 +15,51 @@ env:
     # The next declaration is the encrypted COVERITY_SCAN_TOKEN, created
     #   via the "travis encrypt" command using the project repo's public key
     - secure: "oYIV1EmOvP2ruIxyCTPuGuqUsUzTxxu3P+IdlQZgV6aXaBxtFCuoQaLVMOlnx8xSI2V4mb5I/wK5fAtgUAomzDHQmaBNSbJc3xBTy2xgckup60ehKtFqf+ifm+AYcLQQgLtMUDmLNNcJIKUGPZ8GYwjNfWKt3VGuLHV+UFiRuQI="
-    - coverity_scan_run_condition='\( "$CC" = gcc \) -a \( $USECMAKE -eq 0 \) -a \( "$TRAVIS_OS_NAME" = linux \)'
+    - coverity_scan_run_condition='\( "$CC" = "gcc" \) -a \( $USECMAKE -eq 0 \) -a \( "$TRAVIS_OS_NAME" = "linux" \)'
     - coverity_scan_script_test_mode=false
+    # Various conditions where the Travis CI scripts should behave differently
+    - is_coverity_branch='\( "$TRAVIS_BRANCH" = "coverity_scan" \)'
+    - is_valid_config='\( ! '"$is_coverity_branch"' \) -o \( '"$coverity_scan_run_condition"' \)'
+    - is_valid_osx='\( "$TRAVIS_OS_NAME" = "osx" \) -a \( '"$is_valid_config"' \)'
+    - is_valid_linux='\( "$TRAVIS_OS_NAME" = "linux" \) -a \( '"$is_valid_config"' \)'
+    - is_notcoverity_autot='\( ! '"$is_coverity_branch"' \) -a \( $USECMAKE -eq 0 \)'
+    - is_notcoverity_cmake='\( ! '"$is_coverity_branch"' \) -a \( $USECMAKE -eq 1 \)'
   matrix:
     # Let's test both our autoconf and CMake build systems
     - USECMAKE=0
     - USECMAKE=1
 
-# Only test gcc with autotools, not CMake.
-# We now have three builds: autoconf+gcc, autoconf+clang, CMake+clang
-# Note: In the other xoreos repositories, we use gcc for the CMake test.
-#       However, for the main xoreos repository, it fails to link in
-#       the Travis CI Trusty worker, for some reason. It does work on
-#       other systems, so it might be an intermittent gcc bug?
 matrix:
   exclude:
-    - compiler: gcc
+    # On Linux, we only need to test one compiler with both autools and CMake.
+    # This gives us three builds on Linux: autotools+gcc, autotools+clang, CMake+gcc
+    - compiler: clang
+      os: linux
       env: USECMAKE=1
-    # Exclude gcc build for osx
+    # On macOS, gcc is deprecated, so we only build with clang there.
+    # This gives us two builds on macOS: autotools+clang, CMake+clang
     - compiler: gcc
       os: osx
 
-# If building on osx, use brew to install dependencies
 before_install:
-  - if [ $TRAVIS_OS_NAME = osx ]; then brew install sdl2 openal-soft freetype mad faad2 libvorbis xvid zlib xz libxml2; fi
+  # Installing macOS dependencies, using brew
+  - if eval [ $is_valid_osx ]; then brew install sdl2 openal-soft freetype mad faad2 libvorbis xvid zlib xz libxml2; fi
+  # Installing Linux dependencies, using apt-get
+  - if eval [ $is_valid_linux ]; then sudo apt-get update -qq; fi
+  - if eval [ $is_valid_linux ]; then sudo apt-get install libboost-all-dev libsdl2-dev libopenal-dev libfreetype6-dev libmad0-dev libfaad-dev libvorbis-dev libxvidcore-dev libgl1-mesa-dev zlib1g-dev liblzma-dev libxml2-dev; fi
 
 script:
-  # Configure, autotools
-  - if [ \( "$COVERITY_SCAN_BRANCH" != 1 \) -a \( $USECMAKE -eq 0 \) ]; then ./autogen.sh; fi
-  - if [ \( "$COVERITY_SCAN_BRANCH" != 1 \) -a \( $USECMAKE -eq 0 \) ]; then ./configure || cat config.log; fi
-  # Configure, CMake
-  - if [ \( "$COVERITY_SCAN_BRANCH" != 1 \) -a \( $USECMAKE -eq 1 \) ]; then cmake .; fi
-  # Compile
-  - if [ "$COVERITY_SCAN_BRANCH" != 1 ]; then make -j4; fi
-  # Unit tests, autotools
-  - if [ \( "$COVERITY_SCAN_BRANCH" != 1 \) -a \( $USECMAKE -eq 0 \) ]; then echo "" > test-suite.log; make -j4 check || cat test-suite.log; fi
-  # Unit tests, CMake
-  - if [ \( "$COVERITY_SCAN_BRANCH" != 1 \) -a \( $USECMAKE -eq 1 \) ]; then make -j4 check; fi
+  # autotools build system: ./autogen.sh && ./configure && make && make check
+  - if eval [ $is_notcoverity_autot ]; then ./autogen.sh; fi
+  - if eval [ $is_notcoverity_autot ]; then ./configure || cat config.log; fi
+  - if eval [ $is_notcoverity_autot ]; then make -j4; fi
+  - if eval [ $is_notcoverity_autot ]; then echo "" > test-suite.log; make -j4 check || cat test-suite.log; fi
+  # CMake build system: cmake . && make && make check
+  - if eval [ $is_notcoverity_cmake ]; then cmake .; fi
+  - if eval [ $is_notcoverity_cmake ]; then make -j4; fi
+  - if eval [ $is_notcoverity_cmake ]; then make -j4 check; fi
 
 addons:
-  apt:
-    packages:
-      - libboost-all-dev
-      - libsdl2-dev
-      - libopenal-dev
-      - libfreetype6-dev
-      - libmad0-dev
-      - libfaad-dev
-      - libvorbis-dev
-      - libxvidcore-dev
-      - libgl1-mesa-dev
-      - zlib1g-dev
-      - liblzma-dev
-      - libxml2-dev
   coverity_scan:
     build_script_url: https://raw.githubusercontent.com/$TRAVIS_REPO_SLUG/$TRAVIS_BRANCH/.travis_coverity.sh
     project: