string(APPEND) is too recent for travis's cmake (or 2.8.11, for that matter).

.travis.yml

@@ -11,7 +11,6 @@ script:
   - if [ "$TRAVIS_OS_NAME" == "linux" ]; then sudo apt-get install libgtk-3-dev -y || sudo apt-cache search libgtk3; fi
   - mkdir build
   - cd build
-  - cmake --version
   - cmake .. -G "Unix Makefiles"
   - make tester examples
   - rm -rf *

CMakeLists.txt

@@ -99,8 +99,8 @@ else()
 		-fvisibility=hidden
 	)
 	# don't use C_VERSION or CXX_VERSION because they use GNU standards
-	string(APPEND CMAKE_C_FLAGS " --std=c99")
-	string(APPEND CMAKE_CXX_FLAGS " --std=c++11")
+	set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} --std=c99")
+	set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} --std=c++11")
 
 	set(_COMMON_LDFLAGS
 		-fvisibility=hidden