Lucas, I don't understand too much in the compilation. It seems like all the postfix binaries contain some static library (*.a) instead of using shared (*.so). That's all what I can suggest.
Output for the shared libraries:
ldd /usr/sbin/postfix
/usr/sbin/postfix:
linux-gate.so.1 => (0xffffe000)
libldap-2.4.so.2 => /usr/lib/libldap-2.4.so.2 (0xb76a9000)
liblber-2.4.so.2 => /usr/lib/liblber-2.4.so.2 (0xb769d000)
libsasl2.so.2 => /usr/lib/libsasl2.so.2 (0xb7686000)
libssl.so.1.0.0 => /usr/lib/libssl.so.1.0.0 (0xb7618000)
libcrypto.so.1.0.0 => /usr/lib/libcrypto.so.1.0.0 (0xb7435000)
libpthread.so.0 => /lib/libpthread.so.0 (0xb741a000)
libmysqlclient.so.18 => /usr/lib/libmysqlclient.so.18 (0xb7150000)
libz.so.1 => /usr/lib/libz.so.1 (0xb713d000)
libm.so.6 => /lib/libm.so.6 (0xb7110000)
libpcre.so.1 => /usr/lib/libpcre.so.1 (0xb70d6000)
libnsl.so.1 => /lib/libnsl.so.1 (0xb70ba000)
libresolv.so.2 => /lib/libresolv.so.2 (0xb70a1000)
libc.so.6 => /lib/libc.so.6 (0xb6f2b000)
libdl.so.2 => /lib/libdl.so.2 (0xb6f26000)
/lib/ld-linux.so.2 (0xb76e8000)
librt.so.1 => /lib/librt.so.1 (0xb6f1d000)
libstdc++.so.6 => /usr/lib/libstdc++.so.6 (0xb6e55000)
libgcc_s.so.1 => /usr/lib/libgcc_s.so.1 (0xb6e3d000)
So, seems problem not in the libpthread?
Or, may it be "hybrid" case, when some part of postfix use static libpthread, while other is depends on shared libpthread? I have very weak skills, and in the most cases can only use "trial and error" method.