Fix fpclassify with musl libc headers.

musl defines __fpclassify, __fpclassifyf, __fpclassifyl while
the old headers used __fpclassifyd for double rather than
__fpclassify.
Fix seekdir() for 64-bit off_t.
Fix ftello() for 64 bit off_t return value.
Provide better impl of __assert_fail().
Bump version for new libc headers.