Fix the bug in following scenario:  

gmock is downloaded successfully while gtest not. Resuming "autogen.sh",
the gtest won't be downloaded. And following error will occur:  

$ ./autogen.sh
+ autoreconf -f -i -Wall,no-obsolete
aclocal: error: configure.ac:1: file 'gtest/m4/acx_pthread.m4' does not exist
autoreconf: aclocal failed with exit status: 1