globus-toolkit (set minimum mac os deployment target to 10.5)

If not set explicitly, globus-toolkit compiles libraries with a flat
ld name space. This in turn causes it to pick up the system implementation
of GSS when linked with system framework libraries (as is the case
for the nordugrid-arc package), which causes a segfault as soon as
any GSS related functionality of globus-toolkit is used.

The deployment target is set to 10.5 because it is the minimum
requirement for homebrew.