I found this in the change log of Qt 5.4.2:
- On x86 and x86-64 systems with ELF binaries (especially Linux), due to a new optimization in GCC 5.x in combination with a recent version of GNU binutils, compiling Qt applications with -fPIE is no longer enough with GCC 5.x. Applications now need to be compiled with the -fPIC option if Qt's option "reduce relocations" is active. For backward compatibility only, Qt accepts the use of -fPIE for GCC 4.x versions. Note that Clang is known to generate incompatible code even with -fPIC if the -flto option is active.