Remove version numbering from the pkg-config file.

We don't create include files that are named by version.  Programs linked using -lwiredtiger will follow the symlink to wiredtiger-a.b.c.so, so their referred library name is forever stamped as wiredtiger-a.b.c.so, which won't conflict even when we ship wiredtiger-a.b.d.so .
Refs #1458.

Note: jenkins job wiredtiger-configure-combinations has been adjusted to test building programs using pkg-config (under all tested configurations).