Pass -std=c++11 when using Unix makefile on Travis explicitly

wx-config doesn't include -std=c++11 in its output currently and it's not
clear whether it should (consider c++11 vs gnu++11 or even vs c++14), so add
this compiler option explicitly in C++11 Travis builds.

.travis.yml

@@ -14,13 +14,13 @@ matrix:
           compiler: gcc
         - dist: trusty
           compiler: gcc
-          env: wxCONFIGURE_FLAGS="--enable-cxx11 --enable-stl"
+          env: wxCONFIGURE_FLAGS="--enable-cxx11 --enable-stl" wxMAKEFILE_FLAGS="CXXFLAGS=-std=c++11"
         - dist: trusty
           compiler: clang
           env: wxCONFIGURE_FLAGS="--disable-shared"
         - os: osx
           compiler: clang
-          env: wxCONFIGURE_FLAGS="--enable-cxx11"
+          env: wxCONFIGURE_FLAGS="--enable-cxx11" wxMAKEFILE_FLAGS="CXXFLAGS=-std=c++11"
 
 branches:
     only:
@@ -56,5 +56,5 @@ script:
     - sudo make install
     - echo -en 'travis_fold:end:script.5\\r'
     - echo 'Testing building with the installed version...' && echo -en 'travis_fold:start:script.6\\r'
-    - make -C samples/minimal -f makefile.unx -B
+    - make -C samples/minimal -f makefile.unx -B $wxMAKEFILE_FLAGS
     - echo -en 'travis_fold:end:script.6\\r'