#8 libc_lock mechanism not always available

[Charles Duffy]

On my system (glibc-2.5, amd64, Gentoo), the __libc_lock_* methods are not available. (Defining _LIBC before including <bits/libc-lock.h> results in an attempt to use <lowlevellock.h> and <pthread-functions.h>, neither of which is present).

The attached patch uses pthread-based locking instead.

[Charles Duffy]

[Peter Miller]

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will be fixed in 1.10