krb5 commit: Update Travis build for warning-clean clang build

https://github.com/krb5/krb5/commit/1ae8878998788405bfcd489715008badb69b55ac
commit 1ae8878998788405bfcd489715008badb69b55ac
Author: Greg Hudson <[email protected]>
Date:   Sat Apr 15 21:50:02 2017 -0400

    Update Travis build for warning-clean clang build
    
    Create a custom build matrix which passes -Werror to the clang build
    via a make variable.  (Using a configure variable does not currently
    work, as some of our configure test programs generate warnings.)
    
    Also set the language to C++ (so we use clang++ for the C++ test
    programs and not g++ when compiling with clang), and turn on the
    maintainer-mode checks for the Travis build.

 .travis.yml |   12 +++++++-----
 1 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/.travis.yml b/.travis.yml
index 88ad5e0..59d1da5 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,12 +1,14 @@
-language: c
+language: c++
 
 sudo: required
 
 dist: trusty
 
-compiler:
-  - clang
-  - gcc
+matrix:
+  include:
+  - compiler: clang
+    env: MAKEVARS=CPPFLAGS=-Werror
+  - compiler: gcc
 
 before_install:
   - sudo apt-get update -qq
@@ -21,4 +23,4 @@ before_install:
   - sudo make install
   - cd ../..
 
-script: cd src && autoreconf && ./configure --with-ldap && make && make check
+script: cd src && autoreconf && ./configure --enable-maintainer-mode --with-ldap && make $MAKEVARS && make check