Set Travis CI CC (c compiler) to the toolchain C compiler

The current setup confuses conan profile discovery since conan uses
gcc and clang (instead of g++ and clang++) to ask for the compiler
version. The solution is simply keeping both in sync.

.travis.yml

@@ -8,44 +8,44 @@ matrix:
         apt:
           sources: ['ubuntu-toolchain-r-test', 'boost-latest']
           packages: ['g++-5', 'libboost1.55-all-dev']
-      env: TOOLSET=clang++ BUILD_TYPE='Release' BUILD_DOCS=1
+      env: TOOLSET=clang++ TOOLSET_C=gcc-4.8 BUILD_TYPE='Release' BUILD_DOCS=1
 
     - os: linux
       compiler: gcc
       addons:
         apt:
           sources: ['ubuntu-toolchain-r-test']
           packages: ['g++-4.8']
-      env: TOOLSET=g++-4.8 BUILD_TYPE='Debug' BUILD_DOCS=0
+      env: TOOLSET=g++-4.8 TOOLSET_C=gcc-4.8 BUILD_TYPE='Debug' BUILD_DOCS=0
     - os: linux
       compiler: gcc
       addons:
         apt:
           sources: ['ubuntu-toolchain-r-test']
           packages: ['g++-4.8']
-      env: TOOLSET=g++-4.8 BUILD_TYPE='Release' BUILD_DOCS=0
+      env: TOOLSET=g++-4.8 TOOLSET_C=gcc-4.8 BUILD_TYPE='Release' BUILD_DOCS=0
 
     - os: linux
       compiler: gcc
       addons:
         apt:
           sources: ['ubuntu-toolchain-r-test']
           packages: ['g++-5']
-      env: TOOLSET=g++-5 BUILD_TYPE='Debug' BUILD_DOCS=0
+      env: TOOLSET=g++-5 TOOLSET_C=gcc-5 BUILD_TYPE='Debug' BUILD_DOCS=0
     - os: linux
       compiler: gcc
       addons:
         apt:
           sources: ['ubuntu-toolchain-r-test']
           packages: ['g++-5']
-      env: TOOLSET=g++-5 BUILD_TYPE='Release' BUILD_DOCS=0
+      env: TOOLSET=g++-5 TOOLSET_C=gcc-5 BUILD_TYPE='Release' BUILD_DOCS=0
 
     - os: osx
       compiler: clang
-      env: TOOLSET=clang++ BUILD_TYPE='Debug' BUILD_DOCS=0
+      env: TOOLSET=clang++ TOOLSET_C=clang BUILD_TYPE='Debug' BUILD_DOCS=0
     - os: osx
       compiler: clang
-      env: TOOLSET=clang++ BUILD_TYPE='Release' BUILD_DOCS=0
+      env: TOOLSET=clang++ TOOLSET_C=clang BUILD_TYPE='Release' BUILD_DOCS=0
 
 install:
   - cd ../
@@ -63,7 +63,9 @@ install:
   - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then export CMAKE=$TRAVIS_BUILD_DIR/../cmake-3.3.0-Darwin-x86_64/CMake.app/Contents/bin/cmake; fi
 
   - export CXX=$TOOLSET
+  - export CC=$TOOLSET_C
   - $CXX --version
+  - $CC --version
   - $CMAKE --version
 
   - pip install --user conan && export PATH=$PATH:$HOME/.local/bin