The Mac linker defines flag -headerpad_max_install_names but not all
front-ends recognize the flag and pass it through (many did in the past,
such as the Apple port of GCC). Use the -Wl, option prefix to tell
front-ends to pass it through without trying to interpret it.
We used to suppress generation of -I/usr/include (and on OSX also
-I/usr/local/include). This behavior seems to cause more trouble than
it's worth, so I'm removing it until someone encounters the original
problem it fixed. See issue #8598.