This also cleans up a bunch of things on the way:
-when perl was queried for paths they were not converted to CMake style on
 Windows.
-the result when perl was queried for the perl library name was ignored since
 it was expanded with the possible paths, which is not a valid input for
 find_library(). If perl returns a library name we now will look only for this
 name and not for the default names and use the default names only when the
 executable does not give us a hint.
-get rid of 2 variables that were only used at one place and directly put the
 values in the call to find_library() and find_path().

Inspired by Jeff Trull