Audit places in CMake code where simpified variable expansion is used

[jhendersonHDF]

One intent of PR #5422 was to simplify some places where internal variables in the code get expanded, but the string (FIND ...) command was included in this cleanup though it always treats the second parameter as a string and doesn't try to expand a variable name. Places in the CMake code where this simplified expansion is used to ensure that subtle issues don't exist.