OS X: Only search for install_name_tool if the toolchain has it

When cross compiling, toolchains won't have install_name_tool,
which is provided by Xcode and command line tools on OS X.
This is a Mach-O specific utility and not required on all platforms.
Merge topic 'no-cross-install_name_tool' into next

b3012d20 OS X: Only search for install_name_tool if the toolchain has it