fixes issue caused by commit 1f4e44db0bfdb5449980a11e0bdf06cab602c6b5 when using FTD2XX; reintroduces features of this commit
building of executables is optional now; use cmake -DBUILD_pxarexe=ON to activate
renamed build option to enable compilation of UI (+GUI)
added includes needed to build UI on Linux with gcc 4.7