TravisCI: try: add job for testing with gcc on osx.

.travis.yml

@@ -1,26 +1,32 @@
 language: c
 
-# Default job
+# Default linux jobs
 os: linux
 sudo: required
 dist: trusty
-# Should spawn a linux-gcc job and a linux-clang job
-compiler: 
+compiler:
  - gcc
  - clang
 
-# Aditional testing on OSX with default compiler: should spawn one job
+# Include osx jobs
 matrix:
  include:
  - os: osx
- env: COMPILER=clang
+ osx_image: xcode8
+ compiler: gcc
+ env:
+ - MATRIX_EVAL="brew install gcc@6 && CC=gcc-6" # install and use gcc
+ - os: osx
+ compiler: clang # use default apple clang
 
 before_install:
  - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then sudo add-apt-repository -y ppa:adrozdoff/cmake; fi
  - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then sudo apt-get -qq update ; fi
  - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then sudo apt-get install -y cmake libmpfr-dev ; fi
+ - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then eval "${MATRIX_EVAL}" ; fi
 
 before_script:
+ - if [[ "$TRAVIS_OS_NAME" == "osx" && "$CC" == "gcc" ]]; then export CC=gcc-6 ; fi
  - mkdir sleef.build
  - cd sleef.build
  - cmake -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=../install -DSLEEF_SHOW_CONFIG=1 ..