#14 Building docs fail with out of source build

[Karl Lindén]

When I try to do an out of source build, the build fails with following error:
cp ../../doc/img/* html/
cp: target 'html/' is not a directory
make[2]: [all] Error 1
make[2]: Leaving directory /home/kalle/Sources/dbus-c++/libdbus-c++-git/build/doc' make[1]: *** [all-recursive] Error 1 make[1]: Leaving directory/home/kalle/Sources/dbus-c++/libdbus-c++-git/build'
make: [all] Error 2

This is because Doxygen tries to build the documentation in source directory(?!). The fix is specifying an output direcotry to Doxygen, so that the documentation is generated in the build directory.

I have attached a patch fixing the issue.