It seems that the .pc file is missing the dependency against the
libpthread library. The check.pc.in file seems to include a macro for this, but for some reasons it's not expanded.
To reproduce the problem, do:
1) checkout source
2) ./configure
3) make
4) have a look at check.pc . There no dep for pthread in Libs. There's only: Libs.private: -lrt -lm
This was a distribution problem. Sorry for the noise. The ticket can be closed.
Well, the acx_pthread.m4 macro is quite old (2008).
There is a newer version available at: http://git.savannah.gnu.org/gitweb/?p=autoconf-archive.git;a=blob_plain;f=m4/ax_pthread.m4, maybe you would want to update it.