disk: Remove -mmacosx-version-min from darwin+cgo

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.

disk/disk_darwin_cgo.go

@@ -4,8 +4,7 @@
 package disk
 
 /*
-#cgo CFLAGS: -mmacosx-version-min=10.10 -DMACOSX_DEPLOYMENT_TARGET=10.10
-#cgo LDFLAGS: -mmacosx-version-min=10.10 -lobjc -framework Foundation -framework IOKit
+#cgo LDFLAGS: -lobjc -framework Foundation -framework IOKit
 #include <stdint.h>
 
 // ### enough?