[Regression Tests] Enable matrix eval parameter in setting up compilers

.travis/common.sh

@@ -37,10 +37,11 @@ if [[ $TRAVIS_OS_NAME == 'osx' ]]; then
   # export PATH="/usr/local/opt/qt/bin:$PATH"
   # Install header files in Mojave, if not gcc-4.9 cannot spot stdio.h
   sudo installer -pkg /Library/Developer/CommandLineTools/Packages/macOS_SDK_headers_for_macOS_10.14.pkg -target / -allowUntrusted
-#else
+else
   # For linux, we use g++-8 and gcc-8 as default compilers
-  #export CC=gcc-8
-  #export CXX=g++-8
+  eval "${MATRIX_EVAL}"
+  export "CC=$CC"
+  export "CXX=$CXX"
 fi