Added additional CI test for passing sysroot flags via CC and CXX.

.travis.yml

@@ -32,6 +32,8 @@ matrix:
       env: CXX_BUILD=1 CROSS_COMPILE=x86_64-w64-mingw32- CFLAGS="-D_WIN32_WINNT=0x0501"
     - compiler: gcc
       env: CC=gcc-8 CXX=g++-8
+    - compiler: gcc
+      env: CC="gcc-8 --sysroot=/" CXX="g++-8 --sysroot=/"
     - compiler: gcc
       env: C89_BUILD=1 CC=gcc-8 CXX=g++-8
     - compiler: gcc
@@ -105,7 +107,7 @@ matrix:
 
 before_install:
   - |
-     if [ "$CC" = gcc-8 ]; then
+     if [[ "$CC" =~ ^gcc-8.* ]]; then
        # Install a more recent gcc than the default
        sudo apt-get install -y g++-8
      elif [ "$CC" = clang-6.0 ]; then