Combine the C++ header CI build into the main C build (GH-697)

This will eliminate one of the builds in Travis, allowing for CI overall to complete faster.

.travis.yml

@@ -64,15 +64,6 @@ matrix:
         # Make the `coverage` command available to Codecov w/ a version of Python that can parse all source files.
         - source ./venv/bin/activate
         - bash <(curl -s https://codecov.io/bash)
-    - os: linux
-      language: cpp
-      compiler: clang
-      env:
-        - TESTING="C++ header compatibility"
-      before_script:
-        - ./configure
-      script:
-        - echo '#include "Python.h"' > test.cc && $CXX -c test.cc -o /dev/null -I ./Include -I .
 
 # Travis provides only 2 cores, so don't overdue the parallelism and waste memory.
 before_script:
@@ -88,6 +79,8 @@ before_script:
 script:
   # `-r -w` implicitly provided through `make buildbottest`.
   - make buildbottest TESTOPTS="-j4"
+  # Test for C++ header compatibility.
+  - echo '#include "Python.h"' > test.cc && $CXX -c test.cc -o /dev/null -I ./Include -I .
 
 notifications:
   email: false