TRAVIS: Try suppressing warnings in system includes

For two reasons, SDL2 when compiling with Clang and Boost.Locale on
macOS.

Clang ignores the "/* fallthrough */" comments in the SDL2 includes
and then complains about unmarked switch-case-fallthroughs. The SDL2
search script doesn't mark the SDL2 directory as system.

The Travis CI macOS build VMs have Boost.Locale preinstalled in
/usr/local/ and this is not marked as a system include. The version
preinstalled used std::auto_ptr and Clang notes that this is
deprecated.

.travis.yml

@@ -51,6 +51,10 @@ jobs:
       os: osx
 
 script:
+  # environment
+  - export CXXFLAGS="$CXXFLAGS -O2 -ggdb"
+  - export CXXFLAGS="$CXXFLAGS -isystem /usr/include -isystem /usr/include/SDL2"
+  - if eval [ $is_valid_osx ]; then export CXXFLAGS="$CXXFLAGS -isystem /usr/local/include -isystem /usr/local/include/boost -isystem /usr/local/include/boost/locale"; fi
   # autotools build system: ./autogen.sh && ./configure && make && make check
   - if eval [ $is_notcoverity_autot ]; then ./autogen.sh; fi
   - if eval [ $is_notcoverity_autot ]; then ./configure || (cat config.log; exit 1); fi