build.h changes break qmake build #1076

The recent changes to add a build.h break the build, when building with qmake and using a different build directory from source directory (Qt creator does this by default).

To reproduce:

$ mkdir bitcoin-qt-build-desktop
$ cd bitcoin-qt-build-desktop
$ qmake ../bitcoin
$ make
...
make: *** No rule to make target `../bitcoin/src/build.h', needed by `build/version.o'.  Stop.

I've tried fixing it with the suggestions here, but was unsuccessful: http://stackoverflow.com/questions/5192714/how-to-refer-to-the-source-directory-in-qmake

Ideally, the build process should leave the source directory untouched when building in this way. Other generated source files such as ui__.h, .mocs and .qms also end up in the build direcory, not the source dir. Because of this, _build_dir*/build directory is in the include path, for example. Maybe we could place build.h there?

I had to remove the new lines in the project file, to be able to compile on Windows in Qt Creator, so yes, this needs some more work.

I'm unable to make build/version.o depend on build/build.h, somehow qmake always turns it into ../build/build.h. What is possible, is adding this to PRE_TARGETDEPS, which makes sure it is rebuild right at the start of the build process. That should suffice, but there is a caveat: if your source directory contains a build.h as well, that one will be used. Perhaps we need a build/ directory as well in the non-Qt builds?

EDIT: see my buildinfo branch