[travis] Re-enable clang

.travis.yml

 language: cpp
 compiler:
-  #- clang
+  - clang
   - gcc
 env:
   global:
@@ -20,12 +20,6 @@ before_script:
   - true
 script:
   - NOCONFIGURE=1 ./autogen.sh
-  - echo CC=$CC
-  - echo CXX=$CXX
-  - echo CPP=$CPP
-  - echo LD=$LD
-  - echo CPPFLAGS="$CPPFLAGS"
-  - echo LDFLAGS="$LDFLAGS"
   - ./configure $FEATURES CPPFLAGS="$CPPFLAGS" LDFLAGS="$LDFLAGS"
   - make
   - make check