The presence of the -mmacosx-version-min flag in disk_darwin_cgo.go
makes it impossible to build the other cgo components on modern Mac OS X
(10.12), since the object files with which they must link are not built
with that flag. Errors present from Go Tip (1.9, effectively) in the
form:
ld: warning: object file (whatever.o) was built for newer OSX version
(10.12) than being linked (10.10)
This commit removes the minimum version flag, instead targeting the
version of OS X on which a binary is compiled as the minimum. Without
this, I believe (though have not verified it actually works) that the
only way to build without without warnings/undefined behaviour if the OS
X 10.10 headers and objects are installed and configured correctly.