Revise FindQwt.cmake.

Revise FindQwt.cmake to fix the following:
- in case of error, don't set version="0.0.0" to provoke upstream
  fatal error, but exit immediately with clear error message;
- search qwt.h in all possible combinations of include paths
  and qwt subdirectory names, using CMake's PATH_SUFFIXES parameter;
- try versioned directory and library names first.

refs #9619