Adjusting phoenix.pro to accept shadow build

When trying to install the application after a shadow building
(i.e., 'mkdir build; cd build; qmake ..'), we got errors while installing
image resources (install: cannot stat
‘/resources/images/fritzing_icon.png’: No such file or directory) and
translations (find: `translations': No such file or directory`).

With this fix, we can safely use shadow build.

Note: '$$PWD' is safe to use on the top most project file (.pro), as
stated on [1].

[1] http://qt-project.org/wiki/QMake-top-level-srcdir-and-builddir

Signed-off-by: Rodrigo Oliveira <rodrigo@simb.com.br>
Merge pull request #3025 from rodrigogolive/shadow_install

Adjusting phoenix.pro to accept shadow build