travis: Use Ninja in CMake builds

make does not show compiler command line when -Werror triggers a
compilation error. Ninja does show complete command line in this case,
and the command line might be important for analysing a build error.

Additionally, final build log contains only lines for generated files
and files with compilation warnings, and is much shorter as such.

.travis.yml

@@ -73,6 +73,7 @@ addons:
       # Qt 5.5.1
       - qt55base
       - qt55tools
+      - ninja-build
 
 before_install:
   # only allow specific build for coverity scan, others will stop
@@ -143,7 +144,7 @@ install:
 
       if [ "$build_system" = "cmake" ]; then
         brew unlink cmake
-        brew install cmake
+        brew install cmake ninja
 
         brew install qt5
         brew link --force qt5
@@ -175,7 +176,8 @@ script:
       mkdir build
       cd build
       cmake -DGUI=${gui} -DCMAKE_INSTALL_PREFIX="$qbt_path" "$MY_CMAKE_OPENSSL_HINT" \
-        -G "Unix Makefiles" -DCMAKE_INSTALL_RPATH_USE_LINK_PATH=TRUE ..
+        -G "Ninja" -DCMAKE_INSTALL_RPATH_USE_LINK_PATH=TRUE ..
+      BUILD_TOOL="ninja"
     fi
     if [ "$build_system" = "qmake" ]; then
       ./bootstrap.sh && ./configure $qbtconf
@@ -184,8 +186,9 @@ script:
         sed -i "" -e "s/^\(CXX.*&&\).*$/\1 $CXX/" src/Makefile
         sed -i "" -e 's/^\(CXXFLAGS.*\)$/\1 -Wno-unused-local-typedefs -Wno-inconsistent-missing-override/' src/Makefile
       fi
+      BUILD_TOOL="make"
     fi
-  - make && make install
+  - $BUILD_TOOL && $BUILD_TOOL install
 
 after_success:
   - if [ "$gui" = true ]; then qbt_exe="qbittorrent" ; else qbt_exe="qbittorrent-nox" ; fi