threads: use forward declarations only for glibc

Fixes bug #704908

The declarations of pthread functions, used to generate weak references
to them, fail to suppress macros. Thus, if any pthread function has
been provided as a macro, compiling threads.c will fail.
This breaks on musl libc, which defines pthread_equal as a macro (in
addition to providing the function, as required).

Prevent the declarations for e.g. musl libc by refining the condition.

The idea for this solution was borrowed from the alpine linux guys, see
http://git.alpinelinux.org/cgit/aports/tree/main/libxml2/libxml2-pthread.patch



Signed-off-by: Michael Heimpold <mhei@heimpold.de>