Enhanced libc's pthread configuration detection.

diff --git a/configure.in b/configure.in
index a09d5da..c516dd3 100644
--- a/configure.in
+++ b/configure.in
@@ -643,6 +643,7 @@
 
 AC_FUNC_ALLOCA()
 AC_CHECK_FUNCS(\
+	__libc_fork \
 	_lwp_create \
 	clone \
 	getnetbyaddr \