When using a toolchain with built-in POSIX thread support, static C++ constructors use a static mutex variable which is initialized with `pthread_once` when first used. However, since RIOT's `pthread_once_t` type is different from that in newlib's `pthread`, which is assumed by GCC, RIOT crashes as soon as static constructors are used.
Changing the `pthread_once_t` type to be compatible with newlib's `pthread_once_t` type solves the problem and allows the RIOT `pthread` modules to be used even with toolchains with built-in POSIX thread support.
The pthread header files aren't in the doxygen page anymore after #1137,
because I `@file`'d the `.c` files, not the `.h` files.
This change moves doxygen boilerplate.
Closes#1199.
