added a variable @PROJECT_NAME@_PREFIX and use it as the required variable to make a nicer output when depending projects find this package

removed @PROJECT_NAME@_LIBRARIES from the required variables since the package might not provide a library

Use FindLibrary() cmake command to properly locate the library, even if the directory is already known. This is needed to get the proper full path reference to the library.

fixed issue with compiler/linker flags being interpreted as lists and thus having semicolons in between

${PROJECT_NAME}_CPPFLAGS -> ${PROJECT_NAME}_CXX_FLAGS
${PROJECT_NAME}_LDFLAGS -> ${PROJECT_NAME}_LINK_FLAGS

also make sure that the exported lists are consistently space-separated (sometimes they might be semicolon separated or mixed)