This fixes hopefully the compilation of check_trivstring on cygwin and
solaris. Previously I did not use the explicit instantiation, because c_str()
is used by other explicitly instantiated methiods, but for some reason this
does not seem to suffice.
This is needed since all formats are stored in a global list which is shared
between threads, but never modfified except from the main thread.
The only missing bit is extension_list_, which is not so easy to do.
The interface is now 100% unit tested, and the typedefs depend on the new
STD_STRING_USES_COW configuration variable. The only missing bit is to detect
clang and disable STD_STRING_USES_COW for clang.
As discused on the list. This is not used yet, but it is intended to provide
thread-safe read-access without the need for synchronization if the used STL
implementation does not provide it for std::basic_string. This is the case for
all implementations using copy-on-write.
