Revert LINK_SEARCH_{START,END}_STATIC behavior change

The changes to cmComputeLinkInformation.cxx in commit 675ef165 (Allow
LINK_SEARCH_{START,END}_STATIC props to have default values, 2015-08-07)
are unrelated to the feature added by that change and are incorrect.
They lead to cases of extra -Bdynamic flags.  Revert them.