Fix missing string include error

Found by compiling on os x 10.8.5 by clang 5.0 (500.2.75)
Small refactoring for future
Do not overwrite default flags

Some flags may be setted by default, for example CMAKE_CXX_LINK_FLAGS have
'-Wl,-search_paths_first -Wl,-headerpad_max_install_names'
Fix uri_parse.cpp compilation error

Template instantiation depth need to be increased (add -ftemplate-depth=1024)
Remove unnecessary linker flags

Linker don't use '-std=c++11' flag. To link libcxx '-lc++' flag must be used.
Merge pull request #34 from ruslo/fix.build

Fix.build