I don't think the pthreads are part of the C99 standard, its in C11 IIRC, so it sounds like the types crept in by mistake.
pthreads aren't in any C ISO standard, but the library is builtin to glibc so probably some of it got into the tags file generated from glibc headers.
The bool type seems to be in it, but bool isn't highlighted when using it.
You can put `bool` into the keywords list in a customized `filetypes.c` file, or do like I do and use C++ filetype for C files.