Set up a build matrix on Travis CI

This will run the different builds in parallel (depending on
resource availability on Travis).

The advantages are
- faster test results
- easier to spot which build type has failed
- shorter test log per build type to parse

.travis.yml

@@ -3,6 +3,10 @@ language: c
 compiler:
   - gcc
 #  - clang
+env:
+  - BUILDTYPE=cmake-make
+  - BUILDTYPE=cmake-ninja
+  - BUILDTYPE=autotools
 before_install:
   - sudo apt-get update -qq
   - sudo apt-get build-dep -qq gnucash
@@ -13,6 +17,7 @@ script: |
   set -ev
 
   # First, do the cmake build using the default Makefile generator
+  if [[ "$BUILDTYPE" == "cmake-make" ]]; then
     mkdir /tmp/gnucash-build-cmake-make
     cd /tmp/gnucash-build-cmake-make
     cmake $TRAVIS_BUILD_DIR
@@ -20,6 +25,7 @@ script: |
     make check
 
   # Next, do cmake again, using the Ninja generator this time
+  elif [[ "$BUILDTYPE" == "cmake-ninja" ]]; then
     mkdir /tmp/gnucash-build-cmake-ninja
     cd /tmp/gnucash-build-cmake-ninja
     cmake -G Ninja $TRAVIS_BUILD_DIR
@@ -27,8 +33,10 @@ script: |
     ninja check
 
   # Finally, do the autotools build
+  elif [[ "$BUILDTYPE" == "autotools" ]]; then
     cd $TRAVIS_BUILD_DIR
     ./autogen.sh
     ./configure
     make
     make check
+  fi