Rework the pthread-stub design

The current design handles the most common use-cases, although it causes
breakage on others (when a pthreads liked library is dlopened). Refer to
the README for further details.

The new design, makes pthread-stubs a "meta" package which _never_
provides a library but only a .pc file.

pthread-stubs checks if the run-time (libc or otherwise) expose
lightweight pthread symbols to link against and defaults to a full blown
pthread.

This way projects can use the Cflags/Libs without having to know the
details. Alternatively they can directly link against the pthread
implementation, although that might bring unwarranted overhead.

v2:
 - Remove m4 macro, always use -pthread and document why.
 - Sort the symbol list, document how it's derived what is allowed and
what not.
 - Rework the README to start from current state of afairs to past ones.
 - Document platforms that are 'safe' and ones that are not.

v3:
 - Add SVN note about -pthread + Cygwin/mingw/mingw-w64

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Acked-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Uli Schlachter <psychon@znc.in>