Added -l to everything that seems to lack -l in file that generates opencv.pc

On my three linux machines, after following the 2.4.6.0 instructions (building the library itself worked fine), I would repeatedly get these errors when I try to build programs that link opencv an use ```pkg-config opencv --libs```

```
g++: rt: No such file or directory
g++: pthread: No such file or directory
g++: m: No such file or directory
g++: dl: No such file or directory
```

I believe this is due to ```opencv.pc``` getting created with ```rt pthread m dl``` where g++ expects to see ```-lrt -lpthread -lm -ldl```.  The change in this PR results in the opencv.pc file I expect to see, and building works fine after this.

Could this issue be what's behind these SO questions?  Same behavior I was seeing.
http://stackoverflow.com/questions/10526124/basic-build-issue-regarding-libs-pkg-config-and-opencv
http://answers.opencv.org/question/5391/problems-with-static-opencv-243-and-pkg-config/
http://answers.opencv.org/question/19394/after-installing-opencv-2900-i-cant-compile-any/

Thanks!