Tweak ccache to get legible diagnostics

ccache 3.2 (from Ubuntu Xenial) defaults CCACHE_CPP2 to off (this
setting defaults to on starting from ccache 3.3). That default leads to
unlegible compiler warning outputs because GCC and Clang will emit
warnings using the preprocessed output. Turn it on here.

.travis.yml

@@ -95,6 +95,14 @@ python:
 before_install:
     - eval "${OVERRIDE_CC}"
     - eval "${OVERRIDE_CXX}"
+    # ccache 3.2 (from Ubuntu Xenial) has CCACHE_CPP2 default to off (this
+    # setting defaults to on starting from ccache 3.3). That default leads to
+    # unlegible compiler warning outputs because GCC and Clang will emit
+    # warnings using the preprocessed output...
+    - |
+      if [ "${TRAVIS_DIST}" = xenial ]; then
+        ccache --set-config run_second_cpp=true
+      fi
     - |
       if [ "${TRAVIS_COMPILER}" = clang ]; then
         case "${TRAVIS_OS_NAME}" in