Did you look at GNU libc?
On my Debian box, I see
edd@mccoy:~$ dpkg -S /usr/lib/libpthread.so
libc6-dev: /usr/lib/libpthread.so
edd@mccoy:~$ apt-cache show libc6-dev
Package: libc6-dev
Priority: optional
Section: libdevel
Installed-Size: 13172
Maintainer: GNU Libc Maintainers <debian-glibc@lists.debian.org>
Architecture: i386
Source: glibc
Version: 2.7-18lenny4
Replaces: man-db (<= 2.3.10-41), gettext (<= 0.10.26-1), ppp (<= 2.2.0f-24),
libgdbmg1-dev (<= 1.7.3-24)
Provides: libc-dev
Depends: libc6 (= 2.7-18lenny4), linux-libc-dev
Recommends: gcc | c-compiler
Suggests: glibc-doc, manpages-dev
Conflicts: libstdc++2.10-dev (<< 1:2.95.2-15), gcc-2.95 (<< 1:2.95.3-9),
binutils (<< 2.17cvs20070426-1), libc-dev
Filename: pool/updates/main/g/glibc/libc6-dev_2.7-18lenny4_i386.deb
Size: 3387248
MD5sum: 9cdb8401cead8afdd9f3e5da7bf673f8
SHA1: 01049f981a24dc0e5da8a3a1e57a8f2e91d7e57a
SHA256: 89c222ece39c417ee91f24d9f5619f58be330b111092379e4f09b93bcbfe834b
Description: GNU C Library: Development Libraries and Header Files
Contains the symlinks, headers, and object files needed to compile
and link programs which use the standard C library.
Build-Essential: yes
Tag: devel::lang:c, devel::library, implemented-in::c, role::devel-lib, suite::gnu
edd@mccoy:~$