The fix in commit v3.5.0-rc1~27^2 (FindPkgConfig: set standard variables
in the cache, 2016-01-20) added the wrong variable name to the cache.
The test was only testing that the cache variable existed, not that it
also had the correct value. Update the test to ensure that the cache
value matches the local variable value.
Reported-by: Bernd Lörwald