Remove visibility settings from boost.mk

Clang gave a visibility error when compiling boost with visibility
hidden.

contrib/depends/packages/boost.mk

@@ -20,7 +20,7 @@ $(package)_archiver_$(host_os)=$($(package)_ar)
 $(package)_toolset_darwin=darwin
 $(package)_archiver_darwin=$($(package)_libtool)
 $(package)_config_libraries=chrono,filesystem,program_options,system,thread,test,date_time,regex,serialization,locale
-$(package)_cxxflags=-std=c++11 -fvisibility=hidden
+$(package)_cxxflags=-std=c++11
 $(package)_cxxflags_linux=-fPIC
 endef