extconf does not work with custom lib dirs.

[alvaromat]

I've compiled or-tools and installed into the system. Or-tools is installed into /usr/local. Some libs are placed into /usr/local/lib, while others are /usr/local/lib64. libprotobuf.a is on lib64.

I used flags --with-or-tools-dir=/usr/local --with-or-tools-lib=/usr/local/lib:/usr/local/lib64 to install the gem, but it doesn't work because extconf tries to find the library in a directory that doesn't exist (/usr/local/lib:/usr/local/lib64/libprotobuf.a).