lib: Add thread-safety to global list of handles.

This commit uses the Gnulib 'lock' module to implement a mutex on
the global list of handles which is stored by the library.

Note that Gnulib nicely avoids explicitly linking with -lpthread
unless the application program itself links to -lpthread.  Locks
are only enabled in multithreaded applications.

$ ldd src/.libs/libguestfs.so.0.217.0
	linux-vdso.so.1 =>  (0x00007fffcb7ff000)
	libc.so.6 => /lib64/libc.so.6 (0x00007f96a4e6c000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f96a544d000)

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1 files changed, 1 insertions, 0 deletions

diff --git a/bootstrap b/bootstrap
index 0f8bf20f..6007e597 100755
--- a/bootstrap
+++ b/bootstrap
@@ -66,6 +66,7 @@ gnumakefile
 hash
 hash-pjw
 ignore-value
+lock
 maintainer-makefile
 manywarnings
 netinet_in