fix string size check and assert condition

1. std::string::size() return size_t (is never < 0)
2. "event must be larger than 0" and 'event' has unsigned int type
closed #4048:fixed LabelTTFMultiline test case show nothing on mac.
closed #4054: Removing child from TMXLayer may cause crash.
closed #4054: Adds test for removing child from TMXLayer.
fix string concatenation

set(CMAKE_CXX_FLAGS ${CMAKE_CXX_FLAGS} "-std=c++11") which leads to list of strings.
The result is "-stdlib=libc++;-std=c++11"  if CMAKE_CXX_FLAGS is defined as "-stdlib=libc++"
Merge pull request #5372 from dumganhar/iss4054-tmxlayer

closed #4054: Removing child from TMXLayer may cause crash.
Merge pull request #5369 from Dhilan007/develop_labttfMultiline

closed #4048:fixed LabelTTFMultiline test case show nothing on mac.
Merge pull request #5374 from justmao945/develop

Correcting the definition of CMake variables.
Update AUTHORS [ci skip]
Merge commit 'refs/pull/5289/head' of git://github.com/cocos2d/cocos2d-x into merge5289

Conflicts:
	cocos/2d/platform/win32/CCImage.cpp
Merge pull request #5375 from dumganhar/merge5289

Merge PR #5289: fix string size check and assert condition
Update AUTHORS [ci skip]