The Mac package is being compiled (by me) on OS X 10.8.5, however it was crashing on 10.6.
The GUI seems to be working just buy adding the macosx-version-min=10.6 along CFLAGS, CXXFLAGS and LDFLAGS.
Lowering to 10.5 only works if the Clang compiler is used. Using GCC leads to a std:: error In qf_filter.cpp, similar to the one described in . The reason to go as low as 10.5 is that Qt also supports this version of OSX (on intel 64 machines).
So, compiling with Clang and setting it to 10.5 compilation wise it works.
The user Alex with 10.6 is reporting  that with the above configuration the GUI is ok but simulations are crashing.
As a side note: using Qt compiled by me also lead to problems to end users. I believe it was mostly due to permission mismatches while deploying the package. I switched back to using the official Qt binaries and this problem seems to be resolved.