Fix a small logic error in qmake bootstrap compiler flags

There was a small logic error in the code selecting the debug/release
C(XX)_FLAGS used when compiling qmake, that could lead to us not
specifying any flags at all.

Change-Id: I5d3c44367d535a17570e3602029b84a02706d624
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>