Set CC explicitly in travis to avoid using different compilers for C and C++ files

Author: DaanDeMeyer

.travis.yml

@@ -28,23 +28,23 @@ matrix:
   fast_finish: true
   include:
     - os: linux
-      env: CXX=g++-5
+      env: CC=gcc-5 CXX=g++-5
       addons:
         apt:
           sources: *sources
           packages: 
             - g++-5
 
     - os: linux
-      env: CXX=g++-7
+      env: CC=gcc-7 CXX=g++-7
       addons:
         apt:
           sources: *sources
           packages: 
             - g++-7
 
     - os: linux
-      env: CXX=clang++-3.6
+      env: CC=clang-3.6 CXX=clang++-3.6
       addons:
         apt:
           sources: *sources
@@ -53,7 +53,7 @@ matrix:
             - g++-5
 
     - os: linux
-      env: CXX=clang++-6.0
+      env: CC=clang-6.0 CXX=clang++-6.0
       addons:
         apt:
           sources: *sources
@@ -63,7 +63,7 @@ matrix:
 
     - os: osx
       osx_image: xcode9.2
-      env: CXX=clang++
+      env: CC=clang CXX=clang++
 
 before_install:
   - git submodule update --init