Use of CMake variables with plural wording in the suffix

A few scripts for [the command "find_package"](http://www.cmake.org/cmake/help/v3.0/command/find_package.html "Load settings for an external project.") provide the relevant CMake variables only in the singular form while others set also variants with a plural wording in the suffix. This fact can be respected in the main build script now.

The content of a CMake variable which contains only one list element [should be quoted in this application context](http://cmake.org/Wiki/CMake/Language_Syntax#CMake_splits_arguments_unless_you_use_quotation_marks_or_escapes. "CMake splits arguments unless you use quotation marks or escapes.") so that special characters will also be preserved.